Yes. You'll still need a valve on the downstairs circuit even if you
parallel it with the upstairs one and use a single room stat for the whole
house - otherwise you won't be able to have HW without downstairs heating.

Agreed - looking at various bits on screwfix's site now.
The programmer Honeywell recommend is the ST6400C - which is the one I
already have.
The existing room thermostat is a digital one. I could re-use this and just
add another room stat upstairs?
(This option would give independant HW/CH operation controlled by the
programmer and upstairs/downstairs temperature control (when CH is on) via
each stat)).

Many thanks for your help. Guess what I'm doing over easter.....

Alan.