I've just acquired a 1971 Clubman automatic in surprisingly original
condition, 48,000 miles which looks genuine.

The car has been dry-stored for over two years and now there's no
forward drive - reverse is fine, but in forward gears I can only detect
the slightest attempt to drive the wheels in the 3 position, and then
not enough to move the car. It was working when put into storage, so
what can be wrong?
The selector cable is moving fine. I've changed the oil & filter and
warmed the engine up to full operating temperature. The only tell-tale
sign is that the oil lamp flashes on and off as I most the selector
through the gears, to reverse and back to forward. Someone hinted there
may be a valve somewhere; any thoughts?