Ross, I have had a similar problem with a starter on a Rover K series engine fitted to a Caterham. When the engine was hot, the starter would engage and then jam. Leave it to cool down for a few minutes and it worked fine again. Being fitted close up under the exhaust manifold, a cooked starter is the usual issue.
However, it turned out that on mine the starter was not mounted exactly square to the flywheel, this out of true position was exacerbated when hot causing the pinion to lock on the ring gear. I think it was the engine mounting flange that was out of true, as I recall they found the new starter was doing the same thing until the flange was trued up. Then good bye starter blues. Hope your starter problems get sorted out OK.
