Originally Posted by j-galap

Turned out to be the chassis grounding wire in the end. The block end of the wire refused to budge so I cleaned it up, gave it a coating of penetration oil and then left it a day. Still wouldn't come off. Tried heating the bolt up too - still nothing - think I need to use an impact gun on it.

Anyway, after that the wire and both connections were both very clean so I decided to test the starter - works like a charm now.

I still need to change that wire at some point but the problem is solved and there's no laboured starting any more. Back on the road again