"Set Square" wrote in message
...
snip

It is far more likely that your lack of flow is due to problems with the
pump. How is this wired - is it simply wired in parallel with the boiler
or
is it connected to the boiler's pump-over-run connection?
--

Yes the pump is connected via a pump-over-run connection. It's looking like
a defective sensor on the boiler, there is no electrical feed to pump.

I starting to suspect this / might / have been the (intermittent) fault
since the boiler was first installed and not the three position valve.......

Thanks to all for the replies.