There is a simple wiring diagram at the bottom of this page:-

http://www.dynamoregulatorconversions.com/wiring-diagrams.php

Magnetic indicators are covered in this thread, starting at Post #9:-

http://hmvf.co.uk/forumvb/showthread.php?48211-Trailer-lighting-do-I-need-indicators-and-brake-lights

Location: Oxfordshire

Hello,
And thank you so much for your prompt response. In answer to the four points you mention:

What happens if the side/head lights are off?
It seems to be happening regardless of whether the head and side lights are on or off.

Does having the ignition on or off make any difference?
They only work when I have the ignition switched on

Has the car been converted to 12 volts?
No the car is still on its original 6 volt system

Does the central pork pie light on the spare wheel cover work as a brake light?
Yes the pork pie light in the middle also works as a brake light

I will also need to check for the little matchbox type thingy underneath when I get home from work later, as I haven't checked that at all (didn't even know it was there!) so again, thanks so much for your help.
Annabel

It would seem to be the switch (The matchbox thingy) causing the problem.