Hi Toby and welcome on in
OK....
tobyn wrote:
- should handbrake be on or off?
Off, and leave it in gear (first gear is best to stop it moving).
tobyn wrote:- how often should I start / idle it?
You have made the cardinal sin of not saying what engine it is
However, the procedure is basically the same for Petrol and Diesel. The following is just my opinion, others may vary. You do not need to run it up very often at all over a 6 month period. I would say once a month. Doing it too often, and for short periods can introduce water (from condensation) and caustic gasses into the engine and exhaust system. So when you do fire it up you do need to let it all get up to full temperature, not just a 5 minute tick over. I would run it until that rad fan has kicked in, then turn it off. That could take 20 minutes, depending on ambient temperature. I know it might annoy the neighbours, but revving it up and down a bit, once hot, would be good.
tobyn wrote: - is it worth driving it backwards and forwards on the drive, every now and then, and pumping brakes etc?
Yes to both

(One reason to do so is it prevents the clutch plates/cover assy and flywheel from getting stuck together).
Also :-
Take the battery(s) out and keep it/them warm and dry, (not on a concrete floor by the way) and charge them up with a smart charger say every 4 weeks.
Keep the bodywork clean and if you can, wax it, or at least use a car shampoo with wax in it.
Open the windows / doors / tailgate on warm dry days to air the interior.
Before it gets too cold and wet, check that it has the correct amount (50/50) of pink antifreeze in it. This is not only for ice protection, but also for anti corrosion protection. Very important that is, especially on the flat 4 petrol motor.
Wedge the fridge door open to prevent mould.
Thinking...............................
Ah, how about storing the bed foams and curtains in the house.
I know, so you get a relationship going with the van, put it all back together one weekend and sleep in it, including making a cuppa in the morning
What else......
Other will be along with more suggestions I am sure
