TRC,
the 100 ft was purely a safety margin I used, that was after being invited by some friendly Fire Bobbies to see what sort of "Boom" would be caused by 50 ltrs of many different types of fuel being "Accidentally " fired, to say the least on most carparks you would struggle to find 100ft of space, but I do take your point.
I used a Landrover Td5 that had the optional air suspension pack, that helped to control wander when pulling a single axled Bowser, and when pulling a twin axle with a greater cap it helped to stop the fluid surge when stopping.
Having said all this, just last week on the M11 a Volvo estate car passed me pulling a single axle bowser marked up "100LL" with no other signs visible ( however it could have been empty, but still a Bomb of immense power) I was travelling at about 65MPH, safety is a state of mind, which I fear the Volvo man hadnt yet switched on!!
Vfr