As at the 1st December 2006 the rules changed. Now if you are part of a multi-crew your Class 1 medical is valid for one year, if you operate as a single crew then it will only be valid for 6 months. You will obviously need to ensure that your ECG & Audio. etc are all up to date and in in sync.
It is all on the CAA/SRG website.
Hope that helps.
CC