I would keep my mouth shut until they ask for it back, but dont spend it for a while.
This happened me in 2 different jobs (one 10 years ago and one 7 years ago) and neither of them realised the mistake so I just kept the money and said nothing!
Could it not be a tax rebate? Given that the tax year ended 05/04/07 they may have deducted too much tax from you. If it were me, i'd keep quiet, take the money & run!