Another idea that I’ve seen on an MGB. Starter and pinion ring gears wear especially on their leading edges creating high spots. When hot the high spots caused the gears to mesh too tightly. I theory the starter has a fixed location but I found loosening the mounting bolts and re fixing the whilst applying a little outward pressure to the starter gave just a tiny amount of free play and solved the problem. Similar method to tightening a fan belt but attempting to reduce pressure rather than creat it. Worked for me.

2012 Plus 4 in Sport Green. Much comfier than the Plus 8!