o Based upon the FAQ, I believe the system is a Type C or S-plan
installation

You have an 'S' plan system.

The main unknown is how to wire the unit into the existing system.

Basically, there are four wires going to the zone valve. (Actually there may
be an additional earth wire, as they seem to be metal cased). Amongst them
should be a brown/blue pair. These control the motor. The blue will be
connected to neutral. The brown will be the call for heat from the
programmer.

To wire in the thermostat, you need to break this brown wire at a convenient
location, preferably at the junction box. Trace the cable back from the zone
valve until you get to the junction box. The two ends of these then form the
switched lines for the programmable thermostat. The receiver may also need a
permanent live and neutral, which you should be able to find in the junction
box too.

You do not need to change the wiring of the boiler itself. This is turned on
by switches in the zone valves when the valve detects that it is open. The
correct way to wire a room thermostat is to control the valve and not worry
about the boiler.

Christian.