Hi again. The conatct details you need are as follows
Bali Budaya Tours. 0361 97 5557 or bulanindah@dps.centrin.net.id
These guys will organise the volcano climb - of Mt Batur and I would also advise you do the Mountain Bike ride with them for an unfrogettable expreience. As comapred to the other one I've read about this one is better and safer. No heavy traffic to contend with.
Steven the owner is Australian and is running a sensational bunch of guys. Ask for Aria if possible - he's such a laugh.
Don't pre-pay over here through a travel agent, you'll pay twice as much. Paay them on the day.
Happy Travels
PS Take a jacket for the climb as it's very cold at that ungoldly hour and can be windy up top. My group froze.