Hi
Your 60 day visa must be used within 90 days of issue then once it's stamped ie on you arrival in Indonesia you 60 day stay begins.
last December there was torrential rain in Bali but it was also very hot this was due to some seismic activity somewhere in the Archapelago and also there was the tailend of a Typhoon.
It rarely rains for a whole day and theres still plenty to do even when it does rain.
Either way it will still be MUCH more pleasant than December in U.K

Cheers laurs