The message
from Mike W W contains these words:

It occurred to me that I could site the zone valve in the return pipe
for the zone, opposite the stop cock which is in the feed pipe for the
zone. Then I can isolate the zone for draining purposes and plumbing
work etc. Would that be okay?

I can't think of any reason why not off hand. If there is I am sure
someone will be along in a minute to say so.

--
Roger Chapman