Spoken to 3 different insurers today including A Plan and all confirmed DOC is not available on any Commercial policy and is extremely rare if any others do.
And even when you have a Fully comprehensive policy which allows driving other vehicles, you will find that almost all restrict your cover on these "other vehicles" to Third Party only. And that means that even if you where covered for other vehicles, you'd still have to make a claim against the bikers insurance and not your own. As other have said, its all plainly written out in your policy.
Don't forget, when you renew your insurance on your van you will still need to declare (and you've already given the game away to them) that you have had an accident.