If i undertsand the responses so far i'm looking at having to replace
the boiler - or have i missed the point?

There's no need to replace the boiler. The type of boiler was just mentioned
in that it supports the idea of a penny pinching attitude by the installer.
Adding a room stat to the existing setup is a very good idea. The best room
for it is the lounge, provided it doesn't have a separate heater, like a gas
fire. In the room with the stat, you must remove any TRVs on the rads and
replace them with lockshield valves that the tenants can't turn off. Ensure
all the other rooms do have TRVs on them.

Christian.