http://www.caa.co.uk/docs/33/LASORS2...ookmarked).pdf
See pages 78-79. As far as I'm aware P/UT can only be logged when undergoing training for a licence issue, i.e. when training for a PPL/CPL with a valid instructor. P1/S can be used under differences training for a new type under the same type rating (e.g. for SEP, doing a checkout on a PA28 after having flown a C152)
However, the issue of check flights - usually carried out for insurance purposes is still a bit ambiguous. I log the hours as P1/S. However I've also logged hours on a new a/c as P1/S, which apparently shouldnt have been done. I'm not going to change it though.