OK when starting from cold symptoms of low battery, slow cranking speed once started and run for a while will restart as normal. On last start-up charging light stayed on about half strength even when reved high. Alternator tested and found to be in good condition Starter removed and bench tested, Battery test shower 12.8v
when starter ran battery dropped by1.6v Starter ran smooth without undue effort.
when reassembled if problem persists what may be the  cause help and suggestions much appreciated

Starter removed, stripped, cleaned and bench tested.

You missed a bit . Seriously though, you're describing a worn brushes problem.

check condition of the positive lead from the alternator to the battery.  Terminals clean, no corrosion?  Terminals intact, no rotted terminals from battery acid or oxidation?  Positive cable intact, no breakage of strands within the cable?

What about battery ground cable.  Good contact with transmission? Good contact with battery?  Cable intact?

Thanks to you both number one best mate with good eyes is on cable testing/cleaning, Starter is with man in large shed, me im on coffee duty  I will post results when finished

That used to happen to my K100, the charge light would glow dimly,

Finally tracked it down to when the (aftermarket) heated grips were on.

bit of a long shot I know, but you just reminded me.

Could be wrong, but isn't this the 'well-known' problem with voltage drop due to corrosion/age on the wire between the warning light and the alternator?

Like this problem ?

That's the one! 

That's... rather like using a sticking plaster instead of fixing the problem at source.

To Conclude, Man in big shed says Starter in perfect working order, Put everything back together Bloody K STARTS AND RUNS PEFECTLY.  i can only think she didn't want to go ridding so threw a wobbly