Hi Will
At the risk of telling you things you already know, I think you have a problem on your hands. The diff from my 1961 Ace 2.6, which to the best of my knowledge is the same ENV unit as yours, was stolen some years ago while the car was in pieces and even with insurance money to spend I had great trouble finding a replacement, in any condition. I can advise you of what not to do, however, namely buy one from a 2 Litre Saloon! I found out the hard way that they are similar but not interchangeable. Probably your best bet is to widen the search and find out what other cars the unit was fitted to, there is bound to be an ENV anorak out there somewhere. The only other application I know about is the Cooper-Bristol racer, and there aren't many of those around!
Good luck.
Norman Burr