Gavin, start with the van jacked up on axle stands, so both wheels are clear of the ground.
Depending on what axle is used, Knott or Al-Ko dictates how to adjust.
The Knott system has an adjuster nut on the back of the drum, the Al-Ko has two green bungs, the upper one of the 2 should be removed to allow access with a flat bladed screwdriver to the adjusting star wheel.
With the handbrake OFF, adjust the brake on each wheel until it stops the wheel, then back off until the wheel just turns freely.
Always rotate the wheel FORWARD when adjusting or you will activate the reversing mechanism.
When both wheels have been adjusted, apply the handbrake , it should only travel 2 or 3 clicks.