Starter connections look clean and solid, battery connections look clean and solid. New and old batteries need a booster charge (on a trickle charger now).

Away from home for a few days tomorrow to Thursday so next weekend I'll check the inside of the starter motor. Will update on what I find. Thanks for advice so far guys, anything to watch out for with taking the starter apart ('cos if there's a mistake to be made, I'll make it)?

 

The starter motor and the generator are very similar and quite simple machines. They come apart easily. Clean the inside with spray can electrical contacts cleaner [or brake&parts cleaner or WD-40 but not carb cleaner], measure the brushes, test them by hand to ensure they are not getting stuck in the holder. Do you have a factory manual? May be additional instructions there.

 

Actually there is something you need to watch.
That is......... taking apart the starter and paying attention to the brushes and how they come out and go back together.

There are 2 different brands of starter......Prestolite and Hatachi.
I have included the 4 page instructions covering both brands, as I am not sure which you have......... pg

 

If you take whole starter out from primarycase there is spacer in end of axle. Be careful that you don't drop that spacer inside of primary.