I've done some circuit tracing today after checking the wiring diagram for
the fuel system in the Bentley manual and noticing the warm-up regular and
auxilliary air valve connect to the same point for the +12 volt feed which
comes through the fuel-pump relay, and both connect to the same earth point.

I could not for the life of me work out where the earth point was so after
some head-scratching I opted to measure continuity of the earth connections
using one of the battery hold-down studs (the thread makes a nice earth
point for testing!), and found the earth continuity to be fine (less than
0.5 ohm).

The +12 volt continuity was ok (about 5 ohms), but when I started the engine
and measured the voltage presented on the electrical connector pins for the
warm-up regulator and auxilliary air valve, the result was less than 4
volts!

So my next step was to try measuring the voltage between the +12 volt
contact for each device and the +12 volt supply coming from pin 87 of the
fuel-pump relay (using the 2-pin connector wired directly to the relay
socket to hook up one lead from the meter).

That gave a full 13.5 volts when touching the other lead onto the earth
contacts, and about 10 volts when touching the +12 volt contacts.

This is making me think that the problem is the wires running back from the
warm-up regulator and the auxilliary air valve to the +12 volt feed coming
from the fuel-pump relay.

Does this seem like a sensible deduction so far?

Regards,

Craig.