I'm in the process of buying a late 60s semi, and the homebuyer's
report has flagged up that the central heating system is old and
hasn't been updated for a long time, and will probably need
substantial repairs soon and/or replacing with a newer efficient
system.

This is our first house, so we're not really sure what kind of costs
that will entail. We can't really ask a plumber to come and give us
quotes because we're not in the house yet. I'm guessing the current
boiler is roughly 1980s, with a hot water tank. We would probably want
to get a nice efficient boiler (Condensing?), plus a new main
thermostat and thermostatic radiator valves put on the existing
radiators.

Does anyone have any idea what the likely cost might be? Just a
ballpark figure, like will it cost us £20, £2000 or £2000000?