I've got pretty much the same van except the Nissan variety and am changing over the 650 tank I have in there to an 850. When you look at the figures the van should just about take it with 2 operatives as well, but it doesn't leave much space for extra gear. Of course you don't need to brim the tank every time you go out and if you do have a big job on you could leave some unneeded stuff at home to cut down on weight. I would say 850 is pretty much the limit though unless your a solo operator but then you probably wouldn't need such a big tank.
Simon.
That depends on which figures you are looking at, and whether or not you read or pay any attention to posts on here regarding payloads, gvw, and weighbridges.
My transit connect has a 900kg payload, i have a 500l tank of water, a couple of poles and not much else, yet when i went over a weighbridge with a full tank i was 40kg under the gvw. My water accounted for just over 55% of my payload.
On a recent post here, pure h20 had his vauxhall combo over a weighbridge. 787kg payload, 350l tank plus ladders and other bits and pieces and he was 95kg under the gvw. The water in that instance is only 44.5% of the payload.
A 650l tank in your primastar, or the renault trafic in question, would account for just over 52% of the payload which is a similar percentage to mine. But an 850l tank will be just over 68% of the payload, so it really wouldnt be wise to fit one, unless you know for a fact that you have the extra 200kg to spare. The only way you will know this is if you have been on a weighbridge with a full 650l tank and both operators.