replying to GeoffB, GeoffB wrote:
In case 4, all that would happen is that the HW would continue to run until it
reaches the temp set by the thermostat -- which is what you would want, anyway.
In case 5, the CH would continue to run until the HW reaches the temp set by
the thermostat, then both CH and HW go off.

My conclusion from all this is that the practical effect of wiring grey to
brown (for the HW, before the thermostat) seems to be minimal. Ok, the CH
would run on for a bit until the HW was hot enough (which it probably would
be, well before the CH was switched off), but that's a fairly minor
consideration. Overall, you really wouldn't notice any difference in the
operation of the whole system.

Anyway, I've wired up the whole system in accordance with the standard S Plan
diagrams, grey permanently live. Plus notes attached to the wires so I know
where they come from and go to. I've also discovered why the installer (30
years ago) didn't follow the standard wiring -- he cut the wires too short, so
couldn't reach the correct terminals (this had baffled me for weeks until I
decided to change it).

So that is the end of the story, hopefully for another 30 years.
Many thanks for all the helpful replies I've received.
GeoffB.

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