go to a broker that deals specifically with window cleaners/WFP equipment.
I used alexander swan. It was more expensive than direct line etc, but apparently unless you get a written statement from the insurance company accepting the vehicle modification (and that IS what this is, dont be fooled) then you arent insured and if you have an accident you'll be up s*** creek.
I found that nearly all the big insurance companies will NOT cover a van with a tank in. You really have to go through a broker.