Okay update! I took the starter out and tested it by attaching batter to positive post and grounding to the casing. The starter fired right up, so then I attached it back to the starter wire that goes to solenoid attached positive post on battery and grounded to casing on starter and the starter fired up.
A better test would have been to leave the starter in place and use a jump lead from battery + to starter terminal. This way you are testing whether the starter can crank a motor rather than just spinning with no load. Using the other jumper from battery - ve to starter body would eliminate any possible grounding issues.
I've noticed the battery cables can corrode inside and under the insulation, even if they look ok from the outside. I guess they are that sort of age now.
