My green button came off leaving the centre, luckily I found it on the garage floor. I looked locally and found $390 just a bit too rich, mototbins has a much cheaper option even considering freight to down under!. I know I could get second hand but given the age of them and the UV down here it may not last. Now my question is :  The screw that holds it to the throttle assembly is that all that is required to change it, i.e. is it simple or is there more to it.

Are you talking about the starter button? If so you can silicon on another plastic button.....I used a black cover that goes on top of bolt head to hide it.

td5, have a look at this thread...

You'll be very lucky to find one of those.
This was my solution
https://www.k100-forum.com/t8541-replacement-starter-button
Also
https://www.k100-forum.com/t1430-starter-button-busted

Thanks all.
I've already ordered a new switch from motobins.
Funny thing, I was looking at the switches and thinking the bike has done buggar all K's but the UV here must take its toll, so I installed relays from Eastern Beaver, more for safety (brighter light) and to take a load off the old switch. Thinking that I would replace them soon ! The other day I was near Munich Motorcycles and I phoned the guy I bought the bike from and asked do I need a spare clutch cable as I was reading the forum and most of you have a spare. Anyway he said no lad, I haven't replaced one on my r80 and its done 218,000, yours is hardly run in. Guess what, the bloody thing snapped the next day!
I must say that motobins are good, my heated grips arrived yesterday and my new switch should come Monday so I'll be all good. Now before I get flamed... it does get cold here in Perth! Honest 
Many thanks for the advice
TD5

TD5

Don't forget the nipple and nylon bush that go at the lever. They fall out when the cable snaps.....and guess what- they don't come with the new cable.

I was lucky enough that the nipple stayed in, Nylon bush? would that be where the pin goes through ?