I think I've found the car with the ex-101 Sqn Merlin with the help of a rolls expert Peter Vacher(thanks for the PM AirArt). Its called the Handlye Special and it still exists! It will be doing runs at an MG Club show in August. It is the one reg'd GH 8803 and does look the mustard. Not a modern custom build but a genuine re-engined 1930 competition roadster. Although some references say it has a "Spitfire" engine I hope the true provenance of its Merlin will be equally appreciated. 121 Ops is a fair record for a Lanc and if this is the last piece of SR-H DV302 left then I think GH 8803 is more than just a classic car, its a memorial to all the many brave aircrew who flew SR-H.
Just got to track down the owner and see if he will let us borrow it for the Tansor crew!