Hi Bob/anyone reading this in the future,

Your memory was pretty good - I unbolted the switch from the slam panel
of the car to allow me a bit more working room and stripped the
insulation leading to it back 6in and also carefully stripped back the
rubber weather sealing where the wires go into the switch.

Hey presto, a security loop, a single wire which appears to do nothing
and 2 wires connected to the swich.  All wires are coloured white.  I
pulled out the loop and the single wire and laid them aside, now facing
a switch with just 2 wires therefore a simple closed/open contact
arrangement.  I established with multimeter that open circuit = alarm
siren on and closed contact (the 2 wires to the switch joined together)
= alarm siren off.

So I simply cut one wire off the switch and taped it up, making the
alarm siren permanantly on.  I then taped all the wires up again so it
looked back to factory, or near enough, with good insulation tape.  I
then fitted the switch back on the slam pannel with a new stainless
bolt.

I could have put a switch on, but I figured I never want the alarm off,
if I do I can simply lock the car by the key in the door which doesn't
arm the alarm (as opposed to using the remote in the key, which locks
the doors and arms the alarm).

Tested the alarm and all is now good, including a chav-warning beep
when I lock/arm it!

Many thanks for the pointers from Bob, whom without I may have just cut
through the security loop!
Greg.