I have always read it that,
If I am qualified on the type under the supervision of an instructor for the purposes of a test/check then I am flying PIC under supervision for the duration of that check as long as the check is passed to a suitable standard. If the check were not to be passed then the hours would be logged as dual as I would not have been qualified to be the aircraft commander.
If I am flying with an examiner/instructor in a helicopter on which I am qualified and current and the examiner/instructor is not there in an instructional capacity for the purposes of licence validation then I am PIC if I am the aircraft signatory. That goes for any 'interim' instruction for example a bit of 'IR' unless I am not IR qualified in which case the instructor would need to be the signatory and I would revert to PIC U/S.
For a check pilot assessing a type for validation, he is there purely for the purposes of assessing the validation of the aircraft under the command of the owner/qualified pilot thus he becomes an observer and the owner/qualified pilot remains the PIC.
If I am not qualified on the aircraft type and flying with an instructor for the purposes of gaining that type rating then I am flying dual until which point I am qualified/solo then I can log PIC U/S if I am not the aircraft signatory flying dual.
It's a minefield but, after 4500 hours of rotary the CAA didn't seem to have a problem with my log book when converting to and ATPL(A).
Trainers/instructors who operate from either seat must do a training familiarisation check and a LHS/RHS check each year to ensure that they are 'comfortable' operating from both seats. (fixed wing airline rules but I am fairly certain they carry across)
Cash Again & Again gotta love em.
p.s. Incidentally I did my last LPC with a lovely chap from the CAA who said they are over budget already this year! So, watch out for cost increases!