[Paul Coleman]

THanks for the posts, fellas. I think it would be better going for a bigger set up - The potential is there then for larger contracts in the future. I've been doing some van insurance quotes and, ironically,  the bigger vans seem to be as cheap, if not cheaper than the smaller ones! Weird!
Dave

Precisely.  It's better to think in terms of what you are likely to be doing in a couple of years rather than what you are doing now.  This is why I have an 800 litre tank in a Renault Trafic (the 2.9 tonne version).  With all my kit in there and a full 50 litre trolley, I do need to underfill it a little bit though.  I know its limits as I checked it on a weighbridge a few months back.
My Trafic is cheaper to insure than my old Escort was (different company though).  Make sure you tell the insurance about the van mount as some companies won't offer cover.