1. The Drayton Tempus 7 seems like a good choice of programmer. But
there's also a similar model, a Lifestyle LP722, which seems similar
BUT it does have a fancy-looking backlit display, which might look
funky in the dark! Are there any differences between these two 7-day
programmers? Recommendations?

Any programmer would do. To be frank, most modern systems could work with a
couple of switches. A modern lagged cylinder can be heated 24 hours day. A
programmable thermostat will handle timing for the heating zones. I fitted a
programmer to mine, but only ever select "Off" or "24H". I have a cylinder
stat, heating the cylinder 24/7 and 3 (shortly to be 4) separate heating
zones with their own programmable thermostats.

2. I'd like to replace the crusty old Honeywell room 'stat in the
lounge with a new Programmable Thermostat. But is there any point now
that I've installed the Tempus 7 programmer?

Yes. Programmable thermostats have advantages that can't be replicated by a
standalone programmer. The advantages include convenient access (i.e. living
room), ability to set a nighttime temperature (i.e. 10C, rather than -273C)
and anticipation of switch on time (i.e. coming on early if it is
particularly cold).

3. Should I install a pipe-stat as well as the Honeywell frost stat in
the garage, to protect the boiler? Or would this be unneccesary with a
properly installed Room stat in the living room (probably still
required for boiler protection, I guess).

You might as well. However, it is particularly important to lag the CH pipes
to the garage. Better still, replace any metal with plastic, as it is less
prone to freeze damage.

4. How the hell does my HW system manage to regulate its temperature
without any zone valves? The Honeywell Cylinder stat certainly seems
to be doing something....

Well, it is a simple upgrade. It isn't foolproof, but is a massive
improvement on a system without the stat.

Basically, with the CH On and the room thermostat activated, the hot water
will get heated anyway (and might get too hot). However, in the summer, with
the CH Off, the boiler will only fire when the hot water cylinder is cold.
This will MASSIVELY improve fuel efficiency and maintain the hot water at
the selected temp. Fitting the extra zone valve (or pump) will improve
efficiency further and prevent overheating of the water when the central
heating is on. It would improve efficiency still further to pump the
circuit, if possible.

5. Can I bang a 2-port motorised valve into this system to upgrade it
to a C-plan? I understand that there are venting safety issues with
doing this.. FWIW, the Water Pump is sited directly beneath the boiler
in the garage.

Yes you may normally. However, this may be dependent on the boiler. Some
boilers require an open gravity loop for heat dissipation, which precludes
using a valve. This isn't very common in gas fired boilers, though.

All open vented boilers require an open unvalved path from the boiler to the
vent. Most will also require an open unvalved path from the cistern outlet
to the boiler not shared with the vent path. What options are available to
you depend on your boiler and its safety features.

Christian.