try stephen eyre at G.T.Insurance, he is very helpful, his number is 0115 9394747, we have started going to him now as helpful and competetive, knows about the fitted tanks, just let him know, just because they havent asked about a tank it doesnt mean you cant tell them, if strapped in - its not a modification, if bolted in it is a modificatiion and its up to you to let them know youve modified your van, hope that helps,
george