Why why why why why why are you all obsessed with telling insurance companies about your tanks?
Vans are built to carry stuff. That's what they do. So long as it's not overloaded I can't see how an insurance company could wriggle out of paying you. Securing your tank is just sensible. How many of you have racks for your poles? Surely by your logic a rack is also a modification? Do you tell them about that? What about fixed reels? Again, by your logic, a modification.
I have fixed tanks, fixed reels, a false floor, and other stuff you'd probably call a modification. Rolled one last year, the insurance company paid out - without any question. That's after the loss adjuster viewed it.
By all means pay the extra - if it gives you piece of mind then the extra couple of hundred quid is probably worth it.
Any one not been paid out because they were carrying a tank of water?