Thanks for the reply.
We think we can rule out the heating zone valve not shutting off 100% due to the fact that this does not effect the downstairs heating. Therefore it could be due to your second point - incorrect connections.
The problem can be controlled to a certain extent by...
The upstairs radiators are getting warm when the heating is off and the hot water is on - the upstairs raditors are getting warm via the return pipe i.e. the return from hot water piping is syphoning back up heating return pipe and vice versa when the hot water is turned off and the heating is...
