From the horses mouth, well the CAA anyway.
the P1 requirement is P1 Captain. However we may have up to 500 hours of those 1500 hours can be P1 Under supervision.
For Sea King HAR 3 operators that means 1000 hours minimum of P1 Captain and 500 hours P1(left hand seat), but don't forget you need 2000 hours total anyway to qualify.
I will get it in writing from the CAA. Spoke a very helpful bloke called Mr White, who gave me the info once he fully understood what I was saying to him.
If I get anything alse I'll let you know.