When we moved I rang around everywhere looking for quotes and Polar were by far the cheapest, much to my surprise. However, we wanted the van on a Staurday, so we had to take 2 days not just one. Polar only charged for one day while everyone else charged for two. Another bonus was that we got a brand new long wheel base transit.
enterprise not the cheapest but i've never had a prob with availability of vans - especially those like the renault master (huge ones with long wheel base)