rsauk,
Using GOOGLE I can find only a couple of reference to an "emergency foot valve"....
Both are in reference to tanker (Lorry?) truck delivery of petrochemicals. The "emergency foot valve" seem to be a spring loaded valve that the operator must stand on for liquid transfer to occur. The location would have to be on the bottom of the truck.
I do not see how this would apply to a stationary tank.
MJC