You need to baffle in both directions.
Baffling side to side will stop the water surging when braking or accelerating, but will do no good for cornering. Likewise, baffling front to back will help with cornering, but not braking.
Both effects are just as dangerous and will lead to instability or vastly increased stopping distances. This can constitute an unsafely loaded vehicle, and if the cops see your van swaying around the road they will certainly pull you over. Worst case scenario is a £5,000 fine.
As Dai said, use lengths of downpipe sawn and jammed into the tank vertically. You should pack in lengths of pipe until there is no room for any more. This will effectively baffle in all directions and will prevent surging very well.
Note: a pre-pump filter is very important as there will undoubtedly be swarf in the tank from the baffles, and this will clog the pump.
-Philip