HEADLINES:

It is Monday evening and I am sitting in the living room of the home of Ann and Alan Keen, drinking a cold beer around their fireplace.

From the downstairs basement, I can hear Ann's piano being played, while upstairs in the master bedroom, Alan's "Holiday Express Classic Train Set" is being put through its paces.

There is, of course, no sign of the controversial married couple dubbed "Mr and Mrs Expenses", even though this £375,000 house in Ann's west London Brentford constituency is officially listed by the Commons Fees Office as their "main home".

As has been widely reported, their primary residence has been a deserted building site for over a year now, lying unfurnished and empty until two weeks ago when a dozen squatters moved in.

The Keens say that the house is being renovated but there has been a dispute with the builder so work has halted.They dispute claims that it is derelict. However, sections of floors are missing, there is no running water, and the squatters say they have to take it in turns to sleep on a mattress in a room caked with layers of dust, the only one that is barely habitable.

But I have not come here to commune with the squatters - charmingly articulate as they turn out to be - but rather as a starting point on my mission to find the calamitous Keens. They have kept a low profile ever since they were pilloried as "the grasping Keens" last summer after it was revealed that they had claimed around £140,000 over five years on MP's expenses for their second London home, a luxury flat in Westminster. What angered voters, though, was not only that their main house was just nine miles from Parliament but that both of them claimed interest on the £520,000 mortgage which, although not against the rules, was regarded by some as an outrageous abuse of the system.

In my journey to hold them to account, I will seek them at their homes, constituency surgeries, Commons offices, parliamentary committee meetings and at a hearing at Brentford County Court. By the end of the week, it will be announced that the couple are to be investigated by the Parliament's standards watchdog over claims that they misused their second home allowance, and Ann, the junior health minister, will have bizarrely stormed out of an all-party meeting when I eventually confront her, calling me "the most repulsive person on the planet".

But to start at the beginning. Apart from a box of Alan's ties (liberated by the squatters) and an incongruous Christmas tree, the Keens appear to have entirely emptied their Brentford abode, so I make the (apparently-exhausting) 40-minute commute into Westminster and visit their nearby flat.

It is a fine summer evening, and I am hoping that Alan, 71, or Ann, 60, will be home. Since the Keens bought this apartment in 2002 for £500,000, its value has risen to £750,000, a windfall that the Keens no doubt hope to use to fund their retirement.

I am eager to ask if they intend to re-designate this as their primary residence, given that they have not lived in Brentford for more than 12 months, and if so, whether they'll be "flipping" and claiming the building costs for doing up their Brentford home?

But it's not a simple case of just pushing their buzzer. John, the concierge, who mans the 24-hour reception desk of this elegant marble-floored 12-storey complex, complete with swimming pool and gym, is well briefed. He stubbornly refuses even to call the Keens on his intercom to say I'm here.

I have already repeatedly asked the Keens' press representatives for a formal interview but, having been rebuffed, I change tack. The next day I call their Commons' offices posing as a constituent and ask to attend their surgeries.

A senior caseworker says: "Ann usually holds surgeries once a month by appointment only but none are scheduled for July or August." When I object that nine weeks is a long time to wait for a 10-minute appointment, I'm reminded that Ann is "a busy minister". But other ministers, such as Sadiq Khan the transport minister, manage to perform their ministerial duties and still hold surgeries once a week, including sessions that are open to constituents without appointments.

Later Alan Keen will write to the Standard, saying: "Ann holds advice surgeries on a very regular basis This will continue throughout the summer months and her Parliamentary office will, as usual remain open. All surgeries are by appointment so that constituents' time is not wasted."

However, earlier this year she was accused of being "bone idle" and a "disastrous and lazy politician" by an 83-year-old war veteran who took her to court for breaching what he claimed was her duty of care to him as his local MP. He lost - because the judge held that MPs have no such duty - but the case touched a nerve with many who say that she has little interest in being a constituency MP.

On the other hand, her husband Alan Keen, a former football scout for Middlesbrough FC and currently MP for Feltham and Heston, has no ministerial duties yet his apparent anonymity has led some to dub him "the Scarlet Pimpernel". Records show that he has spoken only four times in the Commons in the past year, well below average among MPs.

You might wonder how he spends his time and why he needs a personal staff of three, which includes his son, David, to manage his diary. His website reports no activities after January 2008. He boasts about being chair of the All Party Football Group - a group with a penchant for industry-funded jollies - and describes their role as being "involved in the debate [now four years old] as to whether the bid should be made to host the 2012 Olympics in London". It's no wonder he also admits on his website that his career "cannot sink any further - unless I cross the floor of the House!".

By Wednesday, with both Keens proving elusive and their offices ignoring my requests for "a chat" and "to see their schedule of appointments", I head to the one place most MPs worth their salt can be found. At noon, the 432-seat debating chamber fills with hundreds of MPs for Prime Minister's Questions. As Harriet Harman, deputising for Gordon Brown, kicks off, and the serious business of remembering the fallen soldiers in Afghanistan gives way to the standard cut and thrust, there is hardly a seat in the house and I crane my neck expectantly to pick out the Keens.

The Doorkeeper, charged with maintaining security in the Commons, offers to help me out. He points to where Alan sometimes sits on the third bench in the far corner but shakes his head. "Alan's not here, you only see him sporadically," he says. Then he turns his gaze to the middle section where Ann typically sits. "Most ministers turn up for PMQs but Ann's not here, either," he says.

At 6.25pm, I finally come face-to-face with Ann Keen. She has been invited to speak to an All-Party Parliamentary Group meeting for Parkinson's disease at the Commons and I have secured a press pass. She looks much older than the pictures on her website and when she brushes past me, I almost fail to recognise her. But her people recognise me.

Just before Ann is due to speak, a panic-stricken press officer, one of two aides accompanying her, sidles up to me and whispers: "I understand you want to ask a question. Will it be on Parkinson's?" I nod. Minutes later, he's back. "Ann is not doing any Q&A, you can email your questions," he says bluntly.

At the end of Keen's three-minute speech to around 90 people, in which she describes how, as a nurse, she had tended to her own mother who had Parkinson's, I step forward and say: "Evening Standard, just one question. I wonder if you could tell us, Ann, how care has improved since the days in which your mother had Parkinson's?"

But before I am half -way through, Ann is running from the room. As I go after her, she turns to her aide, points in my direction and says: "That man has been trying to get me all week. He is the most repulsive person on the planet."

Afterwards, a witness to her behaviour tells me: "It's scandalous that she thinks she can get away with being so rude. Her behaviour makes you suspicious that she has something to hide."

By Friday though, it is apparent that Ann has bigger problems. First it's announced that the Keens are to be investigated by Parliament's standards watchdog. And at 2pm, they are due in Brentford County Court to obtain an order to evict the squatters.

The squatters - in Mohawks and dreadlocks - arrive accompanied by a boom-box pounding out the theme tune to Star Wars but the case is held up as the metal detector fails to cope with all their tongue, ear and nose pins. To the tune of Yellow Submarine they chant "We all live where the Keens are never seen" and cheer the arrival of the Keens' barrister and offer to spray-paint his wig.

When Judge Stuart Plaskow announces that "this rare case" has been brought against "persons unknown", they object, saying they "are known", and one by one delay the start of the hearing by asking to be joined in the action, each spelling their name for the judge. "That's Glennn with three Ns, m'lord."

Predictably, the Keens stay away and win their interim court order and from today their house is empty again. I email a list of questions as their press officer requested. "Don't hold your breath," she tells me. Surely, though, they can't keep forever dodging questions that are in the public interest?

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