Getting a 13 yr old gravity fed boiler system replaced by a Halstead Ace
High combi boiler system.

Would this require replacing any existing radiator pipework to prevent
excessive noise or leaks due to the increased pressure - currently got 8mm
radiator pipes not sure about underfloor piping?

Also, while they're doing the installation I've asked for them to replace
the 12 radiator valves with thermostatic ones. How long is this likely to
take to do seeing as the system is bled anyway.

Also, the hot water tank is being replaced by a closet radiator (as the
missus wants an airing cupboard still).

Current quote is £1800 including the boiler - does this sound reasonable or
is it only me that thinks I'm being ripped off :-) ?