Section 54 of the Act is applicable when the vehicles in question are legally allowed to be on the road. If over weight, they are illegal and thus insurance can be voided. This can be argued on 2 fronts - a) the vehicles should not have been on the road, and b) that there was an omission in telling the insurer that the vehicle was over weight limit.
It would still be hard to argue that the weight of the van wasn't a contributing factor (unless a plane fell on it) - Did the excess weight result in the momentum being greater than in normal circumstances? Did the excess weight mean that brakes were less effective and thus the vehicle was unable to avoid the accident that others caused? Did the excess weight mean that the driver's action were different in any way than if loaded less.
Its a big legal mess.....

It would still be hard to argue that the weight of the van wasn't a contributing factor (unless a plane fell on it) - Did the excess weight result in the momentum being greater than in normal circumstances? Did the excess weight mean that brakes were less effective and thus the vehicle was unable to avoid the accident that others caused? Did the excess weight mean that the driver's action were different in any way than if loaded less.
Its a big legal mess.....