It was on the French website – the second of the hyperlinks in post #2 – that mention is made of the ex-RAF S.E.5a that was owned by Paul Mantz and was amongst his collection of vintage aircraft that were sold off after his death. It relates that after being acquired by a museum in the USA, it was sold to an English owner in 1985 as G-BLXT and was de-registered in 2000. It says that it is believed to have gone into deep storage at the famous (infamous?) Greenham Common warehouse. Presumably it’s still there?