Basically what I am trying to achieve is I would like to have the odd night away in the van and charge my ebike from the leisure battery. It would only be 1 charge cycle per evening spent and would most likely be lots of driving in between so I think a 110ah battery should be fine.
With an bike, you are almost certainly going to have to use an inverter to get the 12v up to 240v mains, and then use your ebike charger to get it down to the 40v DC (or whatever voltage it is). This inverter is inefficient, so definitely worthy going for a bigger leisure battery to give you capacity headroom.
Pete