Hi Marauder,
I just told the insurance company my name is B.A. Barracus and if they didn`t sort it I would go around there and throw loads of Snicker bars at them
I have a multi vehicle insurance policy and the van is also on this, the broker to arrange this is Bennetts, they are local to me so I can take any vehicle I have to them and show them any mods or non-standard vehicles and they search out best deal for me, so all stays legal.
I think a lot of factors are considered....age, no claims, where you live, garaged, non garaged, convictions, occupation and as stated by AlanN who done the modifications.... ie a few more seats put in and held down with a few self tapping screws is not ideal for the insurance company, where as seats fitted in with reinforcement plates under the floor and some adequate M10 - M12 bolts by a professional is more appealing to them.
I am guessing over the years insurance companies have wised up to some Custom vehicles (done on a weekend by mr bodget) only to be stung for a claim when the vehicle falls apart after going over a pot hole
There are companies out there that will insure anything really, depends on what you want to pay, depending on the mods, some insurance companies may ask for the vehicle to be checked first by VOSA (I think) or some may be happy with a report from a reputable garage.
But when your insurance is due for renewal, phone around and say that the van has the extra seats and they have always been there or were fitted by professional company (helps if you know one as they may ask for letter etc etc) and see what quotes you get..be wary of lying though because if you ever have to claim, insurance companies try there hardest to not pay out using undisclosed info on the vehicle as a get out
Its bit of a minefield, but thats how Insurance companies are now..... will take your money .... but reluctant to pay it out...