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

It is called a 2 port zone valve and is widely available. You could add one
to your existing system as is, although it is better to full "sub-zone" so
that each zone is completely independent.

Use a programmable thermostat in the flat to control the zone valve. The
microswitch outputs from the valve will only be required if you properly sub
zone using S-Plan-Plus or other arrangements.

Christian.