Overlap relief on long-standing business

I've taken over a client with a 31 December year end and the previous accountant and HMRC are unable to supply the overlap figure brought forward(!), and it seems sensible to change the year end this year. I cannot remember how to calculate the overlap profit brought forward i.e.which year's profits to use.

Profits are

y/e 31.12.05 £26k

y/e 31.12.06 £28k

y/e 31.12.07 £22k

y/e 31.12.08 £25k

Which year do I use to calculate the overlap profit proportion?