The message
from Mike W W contains these words:

I'd like to install a valve that will cut off the central heating
supply to one zone within my house, at certain times of the day.

The zone I refer to is part of the house that I rent out as a flat.
The tenant works during the day and is at home in the evenings and
weekends. I, on the other hand, live in the main part of the house and
need the heating on 24/7 for most of the winter, as I work from home.
It seems wasteful to supply heating to my tenant when she's not at
home.

Are such valves available? I've seen the simple elctrically-operated
on-off valves, but do they sell them with programmable timers?

If so, and anyone recommend a good-value one?

I don't know of any programmable valve as such but programmable
thermostats are generally available and it is easy enough to install one
to provide an independent zone. I have a cheap one from screwfix
controlling upstairs and a better one (Danfoss TP75) controlling the
downstairs as I am usually at home all day. Sounds as though you could
do with such a setup yourself as it doesn't make sense to run the house
at the same temperature for 24 hours a day. With the domestic hot water
that makes 3 zones for me but you can, within reason, have as many zones
as you like.

--
Roger Chapman