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Boilers don't even have to be that much. You can get a decent Worcester Bosch combi for about £1k. We are having a pretty beefy system boiler to fill a 300 litre tank and it is costing just under £1.7k all in. Labour to fit it is £900.
For flats/smaller houses with standard combi setups I don't see why a boiler and installation should cost more than £2k.
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New boiler depends on capacity. My Vaillant Ecotec was around £1k, and came to about £2k all-in to have all new radiators and piping fitted too, though my plumber was pretty cheap.
Depends on the value of the house (I presumed we were talking in/near London as it's a London forum), but at least for London, I don't think I'd be arguing about it.
A new boiler is about £5k. Not worth negotiating over?