phd, I think you're wrong (or maybe I've just misunderstood you) when you say "I you were doing a checkride with someone who is not an Instructor on a group owned aircraft or similar for familiarisation purposes, I presume it would be logged as P1?"
As I understand it, the only time that more than one person can log flight time simultaneously (except on a multi-pilot aircraft) is if one of them is an instructor. So if you're doing a check-ride with someone who is not an instructor, he would be P1, and you would not be able to log the time. (Or you could log, and he couldn't, in which case you're in charge, which would seem a bit strange if he's checking you out.) (Someone please correct me if I'm wrong!)
As for logging P/US when flying with an instructor, as I said, that's your choice, and I can't see any reason why you shouldn't. But I would make sure, when asked for you P1 time, that whoever's asking you doesn't mind you including dual time.
FFF
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