I would shop around for quotes. When we replaced our boiler and various upgrades 2 years ago the range of quotes was mind blowing. The cheapest was £2,500 and the dearest £9.600. It did involve relocating the boiler from downstairs and moving it upstairs. In the end the quote we accepted was £2,800 as they came up with the best solution, power flush, new thermostatic valves on 14 radiators etc.