Appendix A page 44 of
LASORS 2008 | Personnel Licensing | Safety Regulation
If you were flying in a single pilot aircraft there is only ever one PIC under JAA regulations.
Therefore you can't log as it more than supernumerary unless it is certified as a multi pilot aircraft.
Supernumerary isn't worth much - you can log it when sitting in a 737 as a passenger technically.
If you were flying a multi pilot aircraft such as a lear etc - you must hold a full type rating to log any hours. If you held an SIC rating these hours do not count of anything.
In summary if you were instructing - great you log it as instruction given and PIC.
If you were being trained you log it as Dual - note NOT PIC.
If you were hour building with another pilot under safety pilot FAA 91.109 malarky then you can't log all of that time as only one person can ever log that time as PIC as there other guy isn't required to be there. I.e if you log PIC he can't log anything other than Supernumerary - if he logs PIC you can't log anything other than supernumerary - Can quite often half your hour building time - as most people don't enter supernumerary - as anyone who reads it will probably laugh at you.
In short a lot of reading to do in the above reference given.