It's a one day job. Parts come to about 1K. How much labour does one add for a day's work?
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The power flush will take at best half a day, with 2 guys working together in unison. I take most of a day for the flush.
I suggest you get British Gas in for a quote, to use as a yard stick. They will be the high end of the pricing, but their quote will include a power flush, and thermostatic radiator valves if you do not already have them. BG will do a high quality installation, and will of course try to sell you a maintenance contract at the same time.
So overall if you want high quality then BG will probably give it to you, but the price will be high. I find that my prices are commonly just over half what BG quotes. You can then get other quotes from local Gas Safe Registered installers. Make sure they bring their identity cards with them when they come to quote, and if they have 'left it at home' tell them to come back when they have found it, or you can look on the Gas Safe Register web site and check their photograph and ID.
Yes, the boiler and SOME parts will be about £1k, perhaps more for the highest quality boiler. Then there are additional parts that will add to the price. You should have a by-pass valve, water filter and possibly a scale reducer if you live in a hard water area. You can have weather compensation which will ultimately reduce the gas consumption but add to the initial cost. You may need a new room thermostat, and if so you can choose a wireless one you can take from room to room. Also you should have all the hot water pipes insulated. None of the above will be included in the cheap Ebay prices unless they say so in the advert. There are very cheap boilers available, but you only want one of those if you are selling your property and moving out within the month. 'Look, there's a shiny new boiler on the wall'. 'Oh, good, that settles it, we'll buy the place'. If you have a quick acting solicitor the boiler may just last until they move in. If you are staying there, you want a decent make. German and Dutch boilers are good, as are some British ones. Italian boilers are like Italian cars. They make loud noises, go wrong a lot and ....er... they do not come in Ferrari Red.
And no one, not ebay or British Gas, nor I, can give you a price without coming round to see the installation. The flue route might be tricky, which could easily add £500 or £1500 to the cost.
I hope that's helpful.