WTF. Dont take this the wrong way, but that does sound very odd and I wonder if there is more to it than your side of the story.
There has to be a trigger that has made the PM start demanding after all this time, esp. since a DCA was clearly engaged by them at the time.
FYI acceptance of payment is not waiver of right to claim – if you send a cheque for part of claimed amount stating that it is tendered in full and final settlement and they bank it, your F&F statement is irrelevant, they can still come for the rest. What you need is something in writing that they DCA accept the payment as settlement.
On face value, the PMs actions are now probably into harrassment territory and before dealing with the money issue I would be sending a businesslike but firm letter stating that you consider yourself harassed and that any further contact other than in writing to your home address and you will without warning be engaging a solicitor to get some kind of court order with costs against the PM etc.