Thanks for the advice,
I managed to get hold of another 6V starter at the weekend and swapped them over. The motor I took off was very stiff to turn compared to the one which was going on and lo, the car started quite quickly 😃
The starter is a bit noisy but at least it starts the car now.
Malc, thanks for the tip on cleaning up the gearbox earth strap, I've cleaned most of the connections but never thought of that one.
Jules, never heard of a hard start relay, but it sounds like I'll be getting one!
Cheers,
John