Mini autobox problem

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?
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Regards, Chris (Please take out my car to reply by plain text email)
---1967 Riley Elf---1978 Mini 1000---1971 Mini Clubman---
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