Martin,
There are quite some differences between FAA and JAA. In general terms, you'll have to be pilot-in-command to log the time as PIC, including (approved) SPIC and PICUS flying. You will want to read the relevant regulations
here . JAR-FCL 2, item 2.080 (c)(1) Logging of flight time, Pilot-in-command, (w/explanatory material), page 1-A-9.
Cheers,