[Tim C]

From your description of the fault, the most likely cause is the dip/main beam steering column switch. The main beam working on flash would suggest that you have 12v feed to the switch. It could be the latching changeover mechanism jammed up or disintegrated inside.

I know type 85's quite well but I think they are pretty much the same looking at the diagrams kindly posted by monkeyboyuk. (chapter 12 part 1 page 12.33)

To prove it, you could bypass the switch.
Root around under the steering column and find the multi-plug that feeds the switch (you might have to remove the column bottom cover).
Unplug it, you should have 12v on the red wire (ro - pin 30) and on the black/yellow wire (sw/ge pin X) when the ignition is on.
Short out between the red wire and the white/black wire (ws/sw - pin 56a)
Then short between the red wire and the yellow (ge) wire (ge - pin 56b).
If you get dipped and main beam its definately the switch.

Hope this helps.