Eureka!

I think I've found the problem with the Vitesse starting - closer
examination reveals that the bulkhead mounted solenoid is just being used as
a terminal block. Thick black battery wire goes in and another thick wire
goes out to the starter - i.e. the main feed. The starter is a TR6 type with
top mounted solenoid. There's a thick brown wire that comes out of the loom
and also goes to the solenoid, not sure what that is, could be the facet
fuel pump mounted in the boot? Nothing else is connected to the bulkhead
solenoid - I think I'll get some battery cable and terminals to make a new
long lead from battery to starter. I can then remove the bulkhead solenoid
completely.
There's then the red and white normal gauge wire from the loom that goes to
the terminal on the solenoid on the starter - this was a crappy connection
that I remade last time I looked at it, now I've found that it's brittle
close to where it goes into the loom, there's a small split in the
insulation, only small but it's let moisture in and the wires inside were so
corroded that they fell apart. I have now replaced that, fitted some heat
shielding (it's right next to the header pipe, one that's unwrapped) and all
is well with the world again.