Hi Mike,
I had just the same trouble on my Rover-engined 1999 +4 a few years ago.
It turned out to be the starter motor in the end, which was drawing too many amps. (Morgan had a bad batch apparently).
I eventually changed it for a 'High Torque' PowerLite unit which solved the problem.
Prior to this, in an effort to get to the bottom of things, I'd had the immobiliser unit professionally by-passed; I'd replaced the crank position sensor; I'd replaced the combined starter relay module; I had the ECU tested; I'd replaced all the earthing straps; I'd replaced the coil pack, plugs and plug leads; and had the battery tested. But still it wouldn't always start. It was really starting to piss me off big-time!
So to save you suffering the same grief, maybe a High Torque starter motor is the answer.
Good luck in getting it sorted.
