There was one under a sheet in the Richmond / Teddington area in the late 70's on a drive near my aunts house. Could it be the same car? I used to pester Uncle to take me down to see it every time I visited. It seemed other worldly to a small lad from Barnsley!
There was also a lovely converted WW2 motor torpedo boat moored on the Thames that I would go and look at too. Ah- the memories!