I agree the idea of a master switch is well useful for maintenance
if nothing else.

But then, that would be entirely different. The propsed switch only turns
off the central heating side and certainly doesn't isolate anything. The
boiler should remain powered.

Obviously, some sort of DP isolation should be provided to the entire
system, too. I've ended up with two DP isolators on mine. The boiler and
cylinder are in the loft. The heating zone valves and programmer are in the
understairs cupboard. I have a DP isolator in both locations, both of which
isolate the entire system.

Christian.