I would say that £3000 of that is labour charge ( that's a hell of a lot) a combi boiler to serve 4 /5 rads, one bathroom, average cost £900 up to £1300 at most for bigger boiler, even if you priced each rad @ £100, and top boiler £1300 = £1800. If boiler is going in same place as old boiler, then not to much labour involved, otherwise add a bit more labour, at most £100 per hour. Price your own boiler up, measure your rads, and price them up, not hard to do, then get a one man band to quote you for fitting.