Only three of us still in the area now, but we did some more today and are now well on the way to having a rolling chassis!
1) Got some nuts, bolts and casters from B&Q.
2) Drilled holes for, and mounted gearmotors on their rails.
3) Reinforced the bodywork frames above where we cut out the lower rails so that they could be removed without interfering with the motors.
4) Measured and cut box section pieces to size for the caster mounting points.
Next on the list is completing construction of the caster mounts, and attaching the wheels to the gearboxes - then we have a rolling chassis and can focus on the electronics (the bit which really interests me

).
Drilling holes for the motors in the motor mount rails. Takes quite a while!
Casters from B&Q.
Topping up the batteries - we hadn't actually tested them or charged them since they arrived, so thought it a good idea to make sure they've got some charge in them.
Caster mounts (cut but not assembled). The short lengths of box section will be welded to the main lengths, and the casters attached by 6mm bolts through the box section pieces. Reason for adding the pieces on either side is to stop the casters bending around the rail if they're shock loaded from a fall with the caster pointing sideways.
Chassis with gearmotors bolted on, and caster rails in place to illustrate where they will go. In order to match the height of the rear wheels, we'll need to raise these rails up above the chassis by putting short pieces of box section between them and the chassis.
Reinforcement strip on bodywork frame, positioned above height of motor. These were actually 19" 1U rackmount blanking pieces, spare, from my home network cabinet.
Mockup, showing where the reinforcement strips are in relation to the motors.