Its the first I have heard about an "anti-spill valve" too!
The black "slab" style of tank referred to above is now fitted, saw one a few weeks back.
The old ones were lethal: we smelt fuel, looked back, one metre head of fuel spraying into cabin!
Opened the valve to "dump" it into the main tanks, IMC, diverted into Yeovilton at 5pm Friday, quite unpleasant with the vapours too.
It was after the 1974 accident in field --? (I can see it but not put a number to it!) that the helicopter force was issued with nomex.
lsh