Recommendations for a condensing boiler for 3 bed house - what make should I ask for and any suggestions as to output or questions that I should ask firms when coming round to give estimates. And also, do all radiators need a thermostatic valve - I seem to be getting mixed information.
Thermostatic rad. valves are not technically essential, but well worth fitting if you're having a new system; leave the bathroom rad./towel rail without a TRV, to keep the system stable.
A weather compensating thermostat (sensing outdoors) will help to keep your condensing boiler working at the optimum temperature for efficiency, or you could manually lower the boiler temperature as the weather warms up.
Do you have or want a stored hot water system, with a hot water cylinder, or are you prepared to put up with a combi boiler? The latter usually means a long wait to fill a bath, and more frequent breakdowns. A cylinder is good if you need lots of hot water, as for example a young growing family does. This will affect your choice of boiler. A good boiler, recently recommended by Which? for stored HW systems is the Glow-worm Ultracom 24HXI, £788 retail, but you may need a bigger variant.
Boiler rating (kW output) will depend on the size of your house and how it's built, number of windows, cavity wall insulation, etc., etc. Any competent installer can help you work this out, and should give you a written quotation of the system's capacity. You can specify extra warmth in any given location, e.g. the nursery, or less if you want it, e.g. for the mother-in-law's room. It is a good idea to oversize your radiators, to optimize fuel efficiency.
If you are persuaded to accept a combi boiler, choose a good one, e.g Baxi Duo-tec Combi 28HE (smallest) or Potterton Gold C24HE (smallest), both £800 retail. The exact model will depend on the output you need, probably around 30-35 kW. I cannot recommend any Vaillant boilers, due to past poor experiences, but let your installer guide you if he thinks they've nowadays got their act together.
Get at least two estimates from local tradesmen, especially if you've had them personally recommended; British Gas will probably be way too expensive, and don't go near any rip-off "government-backed" schemes.