I gave purefreedom a ring, brilliant people, very friendly, they said they can supply the frame for me for £299, and they can also call around for a engineering company in my area. I'm going to do it that way. I want it properly done.
@ Nat
Already got the tank, but thinking budget wise, I'd rather save up some more and get it done properly. They say building it in would be around £300.
Got an email back from Cleantech, and this is what they said about the heavy duty tank fitting kit.
The fitting kit bolts do indeed bold thru the floor pan, but there are
10 bolts in total, and each of these has a large metal plate supplied
for underneath the floor, providing more than enough strength for up to
a 640ltr tank (which when full weighs in at 690Kg)
Cleantech
Somehow I don't think my floorpan will hold 30.000kg (400Kg + 66x Gravity @ 40mph accident, not even counting inertia) in an accident, even with spreader plate.