Silverknapper,
I believe I unearthed the answer to your question quite accidentally several months back. I booked in with an AME for a Class 1 medical renewal, not realising that not all AMEs can do the Class 1 renewal (some do the Class 2 renewal only).
IMHO, once the Class 1 medical has lapsed (ie 12 months), it is still valid for Class 2 purposes as the Class 2 renewal frequency is less than that for the Class 1. I was advised, however, that I would have to have my Class 1 renewed within 24 months of the original Class 1 medical @ LGW, otherwise I would have to go through the whole Class 1 again (v. expensive).
I, like you, wanted the assurance that my health was OK before committing to a programme of flying training and so got the Class 1 medical before starting groundschool. However, whilst training, I obviously do not need to exercise the priveleges attached to the Class 1 medical - hence the reason it can lapse.
Please do check all this with an AME (better still SRG at Gatwick).
Final word of warning - when you go onto the CAA's website and search for AME's in your area, note that there is a system for finding out whether that AME is authorised for carrying out Class 1 as well as Class 2 renewals. If the ID no. of the AME starts with a '1', he (she) can do Class 1 renewals. If it starts with a '2', he (she) can only do Class 2 renewals. At least this was the set up last time I looked! By accidentally overlooking this, I booked myself in for a Class 1 renewal with an AME who could only do Class 2s, and managed to inadvertantly saved myself a bit of money.
Good luck!