Having had a scout around the forum,I’ve seen no mention of lithium batts.
Given that the ERIBA vans are about light weight I’m wondering why you wouldn’t fit a pair of lithium batts over a big leisure battery.?

Probably the main reason is that a standard lithium battery is not compatible with the Schaudt onboard charging system. Some manufacturers say that you can use a "gel" setting but even then it is not optimised correctly for lithium. Some lithium-iron-phosphate batteries have built in charge controllers but these can get really expensive, upwards of £600 for one with decent capacity. You can also buy DC/DC regulators to protect the battery when hooked up to the car but it is yet more expense and hassle to install.