85 or 110amp/hour leisure battery. and a split charge relay or some sort.
i run a 100amp relay as the inverter for the ps2 etc is on alot. so the 20amp split charge relay cant cope.
you should get the auto sensing relay so then you dont need to find an ign feed for it.
for a split charge system you need 16 mm2 cable as an absolute minimum, 25 mm2 would be better for a long run fused to 40A both ends, if you use the the thin stuff you will get too much of a voltage drop down the line any you will never fully charge your leisure battery so a 110ah battery will have a far lower usable capacity due to not being fully charged. I give mine a proper charge from time to time on the built in sterling battery charger.
just looked at that kit from brocott, definitely not up to the job, you need to spend more than that on just the cable alone, your van probably has a 90 amp alternator and a flat leisure battery can suck down a lot of current, I would play it safe and make everything with spare capacity rather than running it to its limits.
Take a look at my guide, here.
I used 170amp cable, with 40amp fuses at both ends. I also used a VSR (voltage sensing relay) as it was far simpler to fit than a conventional split charge kit. It did cost more though.
Hope this helps.T: