In reply to Jimmy1990:
It'd also be an idea to look at trips up Meru to acclimatise (assuming you has time and money) - it's a great trip (3 days/2 nights, I think) in its own right, and really helps with the altitude on Kili.
I'd imagine some of the Arusha based companies are contable via email if you want to make contact before you go out, so yes, get a Lonely Planet guide or similar and see what you can find out. Likely to be cheaper than going through a UK company, and if you're out there anyway, not a lot of hassle to sort out.