UPDATE: OK I've some massive backtracking to do here:
I dug out my bank statement which had the Toys R Us transaction on it for Feb last year, then I tracked down the well hidden support number for Playstation (it's 01923 476426, thanks saynoto0870.com) and made the call...
Call answered quickly, I explained the YLOD and the chap said this is no problem and a like for like (i.e. same age and model) refurbished unit will be couriered out on Monday for an 8am-6pm swap. I have to watch the courier open seals and give him my unit
I confirmed the serial number and place/date of purchase, this was all good enough, no fuss no bother - just a cheerful 'thanks thats fine'.
This is EXCELLENT SERVICE - really really impressed. He talked me though what I need to do, I kind of knew this anyway, so being Friday I've the following to do for Monday:
1 - get the sod started as is!
2 - run a backup of everything on there
3 - de-authorise my accounts so my limit of 5 is kept down.
4 - swap my hard-disk out and put back the original
5 - entertain my family on Sunday, they're all over for lasagne.!
Also regarding the 5 user limit, he mentioned that they can only address the limit if it is exceeded, so if I do have a problem getting this started I can call back, should my limit be reached. He mentioned gamesharing as a problem, fair do's.
THE MORAL OF THE STORY: I paid way too much attention to hearsay on forums about Sony needing the receipts, sitting on repairs for months etc. whereas they now have a very efficient swap out service - well done Sony.