Def. sounds like there's something amiss here, instead of looking at your P60 check your December Payslip against your November one and check what the difference is.
However it would seem to me that he has recouped the 120 per month he overpaid you for the first half of 2006 when you weren't actually entitled to it, he should have at least have the courtesy to advise you of this but he also would be entitled to do this if it was an error.
Given your figures I work out if he had done the payroll correct from start of the year and you were to get the 2,500 bonus you should have a gross of €27,000 for 2006
23,000 @ 6mths = 11,500
26,000 @ 6mths = 13,000
Bonus of 2,500
Sound right??