The message
from "Roger Mills" contains these words:

It's wired from permanent live on the switched fused spur that should
run the whole heating system..

How come? The microswitch only closes when the thermostat powers up
the actuator.

The microswitch is closed whenever the valve is open. The valve is
opened by
a motor - when told to do so by the programmer and stat. It is closed by a
spring when the demand is removed.

HOWEVER, if the wet part of the valve jams open such that the return spring
is unable to close it, the microswitch remains closed, and the boiler
continues to fire - regardless of what the programmer and stat are doing.

I am not disputing that as a particular fault and, if you read my
earlier messages, that is exactly what I had previously suggested. What
I was challenging was the NTs suggestion that wiring the microswitch
that way was wrong as it would lead to the whole system being
permanently on. However it seems I may have misinterpreted what the NT
actually meant.

--
Roger Chapman