Gary, it was Jon's earlier post and a phone conversation he had with John Bellinger at AW.
Further, he mentioned that if you did fit one which caused leaks into the electrics, "....Morgan would be well within their rights to decline to cover any damage under warranty."
So I think the way to go is when the car is out of warranty get one fitted.
It was via email, but anyway I get the feeling from that statement that Morgan could or might refuse to cover damage. But there would have to be a very specific leak from the tonneau cover to narrow that down as the problem!