Quick update, my local motor factors supplied me with a replacement starter and wow it's small compared to the original.
I rewired the ignition switch and dribbled some fuel directly into the carb and spun it up.
The engine started on the second go, so it was definitely a dodgy starter motor.
The one I got was an original spec 0.9 kW.
The uprated ones on eBay look identical and are almost the same price. If I'm doing it again I'd go for the starter and alternator combo.
Anyway the car starts and I gave it a quick blat down the road to check the car over, time for an mot now.