I think wazzer is on the button for this one after having a poke around with a torch earlier.
Ironically the car is now behaving itself again (it only did the above consistently this afternoon, never before) and now starts fine, idles fines, settles fine and the code hasn't come back.
I had sat in a motorway tailback for a couple of hours yesterday which may not have helped. Who knows.
I'll keep my eye on it
