Thanks very much, Nigel and Lannis. For now I seem to have solved the issue. Starter interruptus has already been done although I have a longer term ambition to upgrade it with thicker section wiring - just need to get hold of the correct internal spade for the relay end connection.

But, after cleaning up all battery connections, the live connection to the starter solenoid and, I think this might be the thing... removing and replacing the relay a few times to clean the connections, I have a very vigorous starter motor.

But, I shall file away the battery info for the future