Love my bike (don't we all?) but I am getting to the point where I am starting to look longingly at newer bikes on Gumtree (Craigslist in USA).

The thing that is giving me a love hate relationship is 12v power.

I put a fully charged battery in the the old girl (K100RS) and she starts like a champ and off we ride. After a while I start to loose electric stuff. lights, Indicators etc. But she is still riding just fine. I put a meter across the battery and the alternator is is pumping away as it should. If I stop, I cannot start her again. Not enough power to engage the starter. I can bump start her fine and off we go. With no indicators etc.

I have read almost all the posts on here about electrics and there is a lot of talk about starter motor bushes. Can this drain the battery?

Help, I only have half a dozen hairs left on my head but I am getting close to ripping them out!

It sure sounds like starter brushes to me.  The headlight, indicators are all supplied with power via the load shed relay which gets its earth through the starter and if the brushes are cactus then that relay wont operate and the lights and indicator wont work.
If the brushes are low then it can give you the same slow cranking that a low battery gives. Check the voltage when the starter is cranking and if the battery is low the volts will drop way down like 7 or 8 volts but if the battery is good and the starter brushes are the problem then the volts should not go below about 10 volts which is what I expect will be the case.
WARNING Slow cranking of the starter draws a lot of extra current and this can weld the starter relay contacts together and sometimes fry the relay contacts. Be prepared to disconnect the battery in a hurry.