Boxerworks.com

Thanks Kurt - I've just looked at Duanes Website - "BMW motorcycle headlight/ignition switch and keys for the /2 and /5". When the courage comes to remove the switch tabs I will anneal one tab first and see how it goes - I have one of those precision burners which should do the job nicely (borrowed from the kitchen - used for brule or something less important than BMW switch tabs!)
I wish I'd had the sense to buy a spare 20 years or so ago when I did the restoration - they now seem to cost about a million dollars. I'd hazard a guess that in the future the young who want to ride these wonderful machines will have to ignore originality and botch together their own switches - or buy 1970 Hondas!

Removed the switch, checks out fine...all contacts good and the action is crisp. (Incidentally anealing the switch tabs seems to work)
Fully charged battery overnight, it still it ticks over nicely in the ignition position with the red light on then quite suddenly as I increase the revs the headlight comes on!....still the same problem.
The only modifications I've done recently were to add indicators and a new speedometer. Both work fine but I checked the wiring to see if there was anything amiss here that could influence the main problem
So after many days of checking and research I think I'll just think about this in a darkened room and wait for a flash of inspiration....some wily gremlin is hiding in the machine and pops out to turn on the headlights once the revs increase.

Spent the afternoon looking at the wiring then went for a 10 mile ride on the parking lights switch position (I guess they are parking lights as the speedometer doesn't light up in this position) - Battery still fully charged on return (6.4 volts) but I notice the neutral indicator lights up the red generator lens not the green lens so there could be a crossed wire there; the implications of which are not clear to me however long I pour over the wiring schematic....perhaps this is why the headlight keeps coming on in the central switch position...I'll go fishing tomorrow and may have more success...Thanks everyone for being so patient with my ramblings....

I rode 260 problem free miles last week on this bike - on sidelights which works fine.if the wiring/switch is wrong somewhere I can't find it and it can wait for the next rewire in about 50 years time I guess.
I'd post an image but the system for attaching is beyond me....it's probably too simple!
Thanks for all the help and guidance....

SOLVED....As competently advised above .. it was the switch... Terminal 61 is stamped on the switch plate but connected elsewhere on my bike.... As such the adjacent Terminal 58 had been mistaken for 61 at some time and historically I must have attached a loose wire to it which should have been connected to 61.....which explains just about all the problems. Now all the lights and indicators work as they should.
I think next time I rewire I'll use different colours !
Incidentally I found that a small digital camera fits into the headlight quite nicely and on the macro setting reveals the pressed terminal numbers quite clearly so the delicate tabs do not have to be removed to examine the switch.