So, I've flashed my BIOS to F7 and all is well. I wanted to make sure I had the latest update and it mentions 22nm E1 stepping in the list of BIOS improvements. I don't know what E1 stepping means but the flash went fine (I'm typing this using the 2500K).
My 2500K is up for sale on eBum, and I've just ordered the i5 - 3570K Ivy Bridge, which is due to arrive before I have to pull the 2500K out for posting. I was tempted by the i7 - 3770K but I couldn't justify the extra 80 pounds for such a small machine that gets used constantly, but not for gaming. I think I made the right choice, and with the money I get back for the 2500 I reckon the upgrade will only have cost about 40 quid.
I'll reply to this thread when I get the new CPU up and running in case anyone else is wondering about doing the same with a Mini-ITX PC.