Nice clean switch, but still no main beam! Again, I have the hi beam and sidelight, but no main beam. I guess its time to dig out the multimeter and stab some contacts!
Here is a picture of the parts making up a Headlight/Emergency Stop/signal/cancel signal switch for an 88 K75S. There are cheeky springs at the rear of the kill switch that can fly. Also, the headlight switch itself with that tiny ball bearing. The signal cancel has a spring and that brass piston. I did the bag trick and nothing escaped, and checked all continuity during re-assembly. Its a bit dim, but you can make everything out.
How do I get the pic up?

http://hrsbstaff.ednet.ns.ca/bvogel/hi-low_beam_switch/index.html

excellent info here ....thanks to our favorite amphibian.

It may be a silly thing to suggest but have you got 12V going to the bulb?

You say you have hi beam is that when it is in the Hi beam position or when you push it to on the momentary switch?

As Twincarb says about highbeam.............

If you have highbeam, the problem still is in the LHS switch, or the wiring/connectors from the switch to the bulb.

If you only got highbeam flash, the problem more likely is at the RHS switch assy, or the wiring/connectors from the RHS switch to the LHS switch.

here is the information for dismantling the RHS switch if needed

Well, I have a hi beam off the headlight switch at my right hand, no main beam, but there is a sidelight.
At my left hand, I have a hi beam flash, but no main beam.
I have packed up for the night, but will pick it up again on Sat. Working out of town tomorrow.
Thanks for the pointers....I'm not all that "bright" with electrickary!!

Simplified headlamp circuit:

Cleaned everything, connectors/switches etc, and result! All working! Thanks for all the help and advice. Put it all back together today with a bit of spit and polish, and watching West Ham v Man Utd on the box now....perfect!!

Now you just need to pencil in some routine maintenance of cleaning all the other connectors!

Yeah....crossed my mind!
Looking at the overall condition of the loom as a whole, I would say that it probably needs replacing...it all works, but it looks brittle and worn in places. Problem is of course, that when you factor in all the other associated jobs that need doing, you end up talking about a full restoration!!
Only another 15 years to wait until my daughter is old enough to be kicked out, and I can set up in her bedroom!!