Yes that's a good price. I've just paid £2500 for a new boiler (system change) without radiators. It's pretty much what your having done but with an old gas boiler not back boiler.

"run 20 white condense pipe externally to outside drain" 20mm? - My mate told me that Worcester Bosch have just told everyone to upgrade their outside drians to 32mm as they were having alot of problems with them freezing up during the cold weather.