Thanks to all for the replies. I wasn't aware that zone valves could
be controlled by a programmable thermostat/timers. Can anyone recommend
one, or suggest what is a reasonable
price to pay? About £60 - £70?

They're pretty much a commodity item. i.e.
http://www.screwfix.com/app/sfd/cat/...00724&id=29099

Programmable stats are more personal preference. They have different
features, appearances and prices.

So, from what you say, it sounds like I need to make sure
I get one with a microswitch connection.

You misunderstand. All 2 port zone valves will have a microswitch output.
However, you'll only need to wire that up if you properly subzone the
system. Remember that if you don't properly subzone, then the tenant's
heating will only come on when BOTH your heating thermostat demands heat and
the tenant's thermostat demands heat. If you have independent control, then
your heating could be entirely off (i.e. you are on holiday) and their
heating will still work.

Christian.