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?
Regards, Chris (Please take out my car to reply by plain text email)
---1967 Riley Elf---1978 Mini 1000---1971 Mini Clubman---
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
Add image file
Upload is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here. All logos and trade names are the property of their respective owners.