Thank you for your reply.
My firm does operate a PAYE system.
The situation is that I earned much less in the previous tax year than I will in the current and if the payment is allocated to the current, then I will indeed pay much more tax, than if it was allocated to when it was actually "earned" in the March.
My old firm has issued my P60 and has allocated this payment into this tax year rather than last and I am wondering if I can get HMRC to re-allocate to the previous year, so I can reduce my tax bill.
I will definitely press them on this. politely but firmly !
Thanks again for your help