There are nearly three hours of impassioned, richly melodic music in James MacMillan's The Sacrifice, excellently performed by a fine cast. This was appreciated by last night's audience at the Oxford leg of the opera's world premiere tour - culminating next week at Sadler's Wells - who cheered composer, singers and orchestra warmly.

Yet this was hardly an evening without problems. Since it opened in Cardiff in September, critical response has been subdued though one suspects the performance quality has grown significantly since the first night.

The serious problem is the over-wordy, poeticised libretto by Michael Symmons Roberts, one of MacMillan's regular collaborators. The Mabinogian-inspired plot is strong: a marriage takes place across a sectarian divide, arranged in the hope of achieving peace but subsiding into terrible violence.

But each potentially dramatic moment, especially the killing of a child, is rendered null by inflated comment and soliloquy. It's hardly a novelty for a full-scale opera to need adjustment after its first showing. MacMillan's music is stirring and heartfelt. A leaner version could transform it, and provide the tension it now badly lacks.

Lisa Milne, Christopher Purves and Sarah Tynan led an impeccable cast, with top quality singing and playing from the all important chorus and orchestra of Welsh National Opera, conducted by MacMillan. Katie Mitchell's staging, if emotionally occluded, had style. Catch it next week. It's flawed, but this is a huge, eloquent score that deserves a hearing.

• Sadler's Wells, 26 November, 7.30pm, (0844 412 4300).