One of the best and most experienced in the tour guide and cars in Sri Lanka is Ceylon Tours. I think they are the oldest or one of the oldest. Fantastic service and all drivers speak English but also know the ins and outs of taking you around and getting in and out of places.
I have been with them for many times and has always been very good. They always a Rep at the airport 24 hours and I am sure you would enjoy it.
If you want I can give a contact too.