Robert, as I understand it, when towing the van is covered for 3rd Party risk only.
Therefore if it parts company from the tow car and hits someone else, their damage would be taken up by your insurance.
The van, probably a write off by now, would be covered by your own van insurance, if you have any of course.
However, as having separate van insurance is not a legal requirement, I pity anyone, who finds themselves in a position of having a multi thousand pound van destroyed and no replacement policy in place.
Slight edit ,,No I dont pity them, they should hav taken that into consideration in the first place.