I have a 500 litre tank in my LWB Connect and cost me £600.00 from Graig at Crystal Pure Systems,I unbolted the tank frame from my Transit van and placed it into the new van and he just fitted it,he is based in Essex and was pleased with the price,it was either him or to drive up to Grimsby and have it done by Pure Freedom again as this is where I purchased it in the first place.
I think it's far more safer to get it done properly and is money well spent,in reality your life IS at risk!