Presumably you still have the paperwork for the policy, which should contain all the terms and conditions, so you can check what it says about cancellation.
If you haven't, Assure UK have a freephone number - 0800 980 0770 - you could call and ask for a copy of the paperwork.
As for the DCAs, inform them (by fax or letter) that because they have failed to provide full details you are unable to confirm whether the money is actually owed, and you will contact them again after you have taken advice.
The
DCA's contract is with the insurer, not you, so they should look to their client for their fees (unless the policy contract specifically states otherwise).