Old UK heating system

A friend has an old back boiler heating system (Baxi, I think) and an upstairs radiator is leaking - bottom of radiator has rusted through. Due to lack of funds and as the system is so inefficient that they don't use it, they wanted to drain it down or isolate the radiator.
All of the radiators have only one regulator valve, the other side being a plain 90 degree union. There do not appear to be any drain points on the system unless on the boiler behind the gas fire. The system appears to be fed from the same header tank as the DHW system and there is no obvious way to isolate the cold water supply to the heating without also isolating the hot water tank - the only visible wheel valve being to isolate the hot water tank.
The only idea I have is that it any isolating valves may be below the hot water tank which is in a cupboard above and at the end of the bath, the whole bathroom floor BTW is approx 6 inches higher than the rest of the upstairs floor which makes me think that it is to accommodate plumbing.
Apologies for the length of this post. All suggestions gratefully received.
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