Hamish, I had exactly the same sort of check, moving from a PA-28 to a PA-38, the instructor checked me out and told me to log it as P/UT.
The club insurance policy states that if a pilot has not flown with the club for more than 6 weeks, a check with an instructor is required. On this check, the instructor told me he was just a passenger and to log it as P1/S. I looked into this, and told him this was only for exams. He hadn't realised, and told me to log it as P1 instead.
The use of P1/S can be confusing, and I do not think that all instructors know exactly what it means. I think it could be made clearer.