NI contributions when employment status changes

I have a client who is a personal trainer.  For 9 months of the current tax year he was mainly self employed with a couple of classes that he taught at local leisure centres where he was on their payroll; for the last 3 months of the year he has been employed but with a very small number of private clients, so he hasn't completely given up his sole trader business.  What is the procedure for calculating NI due in this situation?  I haven't actually seen his books yet, but I presume he's been paying the flat rate NI contributions throughout the year as a sole trader.

I suspect that he has paid all sorts of strange amounts in PAYE too, since he said that two of the leisure centres had 'asked him for his tax code' when he changed main employer.  Goodness knows what tax code they were using if they were asking the employee for the tax code!!

There's nothing like a challenge!!