Well, I've had some luck.
I got quite a few replies via PeteT16's "partsgateway" link, including one from the Rover Breakers mentioned in PeterZS's post immediately above.
Unfortunately most either went quiet or said "no" when I said that I required the ignition switch transponder, central locking ECU and keyfobs too -apart from the aforementioned Rover Breakers, who wanted ?192 (ouch).
Fortunately somebody local to me recommended this place:
http://www.allcarelectronics.com/
It's quite close to both my home and work addresses - in fact they had a driver in the area late this morning. So they collected the ECU from me, and will rebuild it retaining the original coding and engine map for ?165. Now I know that sounds like a lot compared to ?192 for everything, but bearing in mind that the ECU had already had a remap and replacing that would cost me at least ?150, I'm pretty happy. I should even be able to get the security codes so that I can get another keyfob programmed into it, and as they're nice and local, I've got a few more options should the job not go straightforwards.
With luck the car will be roadworthy again within the week...