I think I have just found the answer ... p132 of CAP 804 has a handy table listing a load of permutations for what should be logged when.
"Pilot undergoing any form of flight test with a EASA or CAA Authorised Examiner (other than case K." is logged as "PICUS" in the "P1" column.
Unfortunately that means that all of the hours I have logged on FAA checkrides don't count ....
Happy days. I'm glad I found that out now rather than when I apply to have my licence issued!