I went and got my class 1 before undertaking ppl(h), got that, went and got ppl(h) and have just let it lapse while doing cpl(h) groundschool, I believe you need it to undertake cpl(h) flight training, if not certainly for cpl skills test.
An itial Class 1 is done at Gatwick, thats valid for 12 months, you can exercise class 2 privilages for 60 months, you only need it revalidating by an approved body. I know a doctor around the corner who will do mine when I need it, far less involved in revalidation than the initial, i.e no ecg's etc on reval, but it all depends on your age.
How old are you Rudiger?