G.T
I had this very problem myself with co-jo hours with a CPL on multi crew aircraft. I did go speak to an inspector at the CAA and basically it boils down to this: your co- jo hours go into your log book as those that you have flown ie the full amount, however when it comes time to calculate your hours for the issue of a higher license, an ATPL obviously, then your co- jo hours will count half of what is in your logbook. So lets say you have 1500 hours total with 200 of those being co pilot then you will in fact have 1400 hours in the eyes of the CAA and will thus have to fly 100 more pic or 200 more as co pilot to be eligible for the higher license.
Hope I have explained it ok.
Cheers C.B