I obtained my initial class 1 last february, so as it is due for renewal soon, I found out about the situation:
1) A class 1 can be used as a class 2 for private flying for 5 years (or at least it can at my age, 23, as most things are valid for at least 5 years)
2) There are then time limits for how long you can wait to renew it back to a class 1 without doing the full class 1 again. It's something like 3 years before you would have to go back to Gatwick to do the full thing again.
I can email you the exact info if you want.