Kevin, you do not legally have to have caravan insurance (strange as it may seem) BUT you MUST inform your car insurance that you will be towing, which may or may not increase your premium.
Whilst actually towing your van is covered for 3rd party risks ONLY, not fully comp as your car may be.
So, if you damage your own van with no one else involved, its your pocket that will suffer.
If you hit someone else with your van, they can claim from your insurance.
If someone else hits you, then you claim from their insurance.
Thats it so far, BUT, if for example, you are at a site , unhitched from the car and your van rolls away, which has happened in the past and will happen in the future, and hits something or someone, then YOU pay all the cost, which can be many hundreds or thousands of pounds.
It is therefore only sensible to have caravan cover arranged through one of the more reputable companies, such as Caravan Club, Camping and Caravan Club etc.
Price, or seemingly low price, is NOT a good idea and make sure you read the small print very very carefully.