If they've connected a charger or tester by the jump point under the bonnet they've wasted their time and yours too as this is only connected to the rear starter battery. The issue will be with the front battery.
Removing the batteries is quite easy, you need 8mm and 10mm sockets on a smaller socket set.
Pull the driver's seat forward as far as it will go. You'll see a plastic cover. This cover pulls out from the front, just under the slide handle. This exposes the batteries. The front one keeps things going (internal lights, central locking etc) when the ignition is off, the rear is connected to start the van and is disconnected when you switch off so it is always ready even if the front goes flat. You should hear a clunk from the relay as you turn the ignition on and off, it is located behind the plastic cover at the back of the seat base.
Disconnect the battery earths (black cable), then carefully take the breather pipes out and push the earths away from the battery terminals. Disconnect the live cables. Use the sockets to remove the two screws and one nut that holds the t-shaped clamp down. The screw on the doorside is on the outside of the seat base, the other screw and the nut are on the inside. Take the clamp out. The batteries are now free to remove.
Reassemble in reverse order, taking care to refit the vent hose in the floor without snapping the Y-piece,. A bit of copperslip on the screws and nut wouldn't go amiss.
The job can be made easier by removing the seat from the base.
It's best to replace both batteries, and swap them over regularly t (say every couple of months) increase longevity as the front battery is always working. You need low height batteries (175mm) a common fitment is the 100 type, usually 70a/hr, but you can also fit 110 type, 80a/hr.
Mat