All Appreciate without sight of the system, it's hard to comment and there LOADS of variable, but I'm after a ballpark figure for replacement of a functioning but tired floor standing Potterton Kingfisher II with a suitable combi. Simplistic system, gravity fed 6 radiators, hot and cold water tanks in same cupboard that backs onto boiler. Boiler to be moved into airing cupboard (literally other side of wall it's on) with horizontal flue length of 3 metres to existing airbrick and there's a suitable overflow to the drain for the condensate pipe. Powerflush as necessary/stipulated, and some TRV's. Apparently gas supply pipe does not require upgrade. Issue is quotes so far have varied from £1445 to an eye watering £6000, and excluding the boiler and replacement rad costs I just can't reconcile them. Bexley based, 2 bed purpose built flat if that makes any odds? Cheers in advance, hugely appreciated.