2 of the RHD ones have been written off (there may be others I do not know of) and as Graham said, Hammond wrote about his accident(s) in Top Gear (I believe he had a few but one was larger than the others).
Weren't two more built to replace them Simon?
I believe one more prototype was built to replace the factory car that was written off. Rumour is that it will never be sold and will end up in the MMC museum (if it survives that long

).
What are the differences between an Aeromax and an Aeromax SS (apart from the £20k price gap)?
The main differences are the scalloped rear end (with accessible deep boot) and the removable targa panel to accommodate open air driving.
The Aeromax strays from traditional Morgan design in that it is only the second fixed head coupe the factory has ever made; the first being the Morgan +4+ which only made it to 28 sold due to a very poor reception, which reflected in low sales. The Aeromax, however, was over subscribed and had a waiting list for people to drop off the waiting list.