The car ended up in the hands of amateur racer Brian Croot, who entered it as an Allard, presumably because the Sphinx name no longer meant anything if it was not being driven by a member of the Sopwith family.
He later raced a C-type Jaguar, and the Allard/Sphinx was put aside. I was in touch with his son Max a couple of years ago, but I can't remember whether they still had the car. I think not. They did still have its old transporter, a Commer Superpoise converted from an old ice-cream wholesaler's truck, and this was slowly rotting away on the family's farm.
(PS I have just found out that Croot fitted the car with a C-Type engine, which is probably why he stopped calling it a Sphinx - there was no Armstrong-Siddeley connection. The car is now in France.)