Thanks for all the advice so far chaps.
I have a friend who works for Walon in Doncaster and may be able to cover the labour for nothing. May even get the part at a reduced rate.
I also have a couple of irons in the fire regarding getting a used HPFP from a source which I know to be reliable, or I could just buy one new.
As for coding/fitting - thanks for confirming what I suspected which is that it should be plug and play. I watched the guys at Statlers change my Golf HPFP and it looked fairly easy (of messy) so I'll consider a DIY.
As for fault codes/diagnostics - to be honest my code reader isn't finding anything and from what Doughboy says, it sounds like shag and hassle for what may be nothing.
If I can get it fixed for anything up to say £250 ish I will just bloody do it. That way, if my problems continue I can sell my part as a working/used part for some other poor bugger and I'll only be about £150 worse off I suppose.
DB how long have I got until it starts failing to start do you think?
PS - something I've not mentioned is that se of the poor running I experienced during hard driving at the Ring seems to tie in with the other symptoms that are often recorded. Could just be convincing myself of that but there you go.