This is the part i ,m never sure of as sharing cost of diesel in 1.9 van fully loaded 424 mile between 4 of us
what do we each pay? and am i best to fill up to start off or fill up halfway would the weight of full tank and fully ladened be more?

To make it fair for all I always start with a full tank & then top up at the end of the journey, so what ever I have to fill up by is what we've used.
I have on many ocassions been over charged by some one guessing.
Yes full tank does add weight but I think to be fair to all it's best doing it this way.

I agree with Gemma. I travel to shows with a couple of friends and we always get into a full car. The car is then filled up at each end and the cost split between us. We do have a long journey as we start from Edinburgh. Though 2 of us start even further away and we share the costs from mine  (I get picked up) to our friend in Edinburgh.