One last think I would suggest is to turn off the power to the heating
system completley (normally via a Fused Connection Unit (FCU) near the
boiler) this should stop the pump and make sure there isn't any power
going to the valve keeping it open (maybe even turn off all the power
in the house to be 100% sure someone hasnt crossed wired it somehow!)

I'll try it. I think everyone here is getting fed up with my 'experiments'
with the CH and HW at the moment, so it will have to wait a while ;-)

You say there is a little leaver on the side

Yes. It looks like a lever rather than a position indicator because there's
a sort of 'gate' arrangement to latch the lever into. Interestingly, it's
not latched, but just back in the what I assume to be 'open' position. So I
don't know if this has been manually set that way or is just indicating the
the valve is open. Sadly t'internet reveals little of these Canadian-made
valves, so no data sheet to check.

try to manually move the
lever, but don't force it and break it!

Sounds like a job for Monday morning. I don't move little breakable levers
on Friday evenings ;-)

Sometimes they stick because
they have been on permanantly for weeks on end (normally because you
have the HW on 24h on the timer, and the thermostat on the side of the
cylinder is set to high (too high meaning higher than the boilers
internal stat)

Interesting. We do run the HW and CH on 24hrs. I'll see if I can determine
the relative settings of the boiler and cylinder stats.

Cheers,

Al.