Well after taking the charging unit out of the van and opening it up it was pretty obvious what the first thing to sort was. One of the capacitors (yellow rectangular one near the smaller pcb connector) had exploded and covered part of the pcb in a tar like substance. I desoldered what was left of it and cleaned the pcb up as best as i could.
I replaced the capacitor and the fuse next to it that it had took out and hoped there wasn't any more damage.
Turned out there was - the track on the pcb which runs from one leg of the capacitor to the mains filter was open circuit, even though it looked ok, so I put a link in on the under side of the pcb to rectify this.
I also found several dry joints on the smaller mains input pcb, which would simply have been caused by age (25 years old!) so I re-soldered these.
Popped it back in the van and jobs a good'un - everything charging as it should and the control panel working as it should.