If I remember from my insurance days you would be covered by the tow vehicles' third party insurance for any damage done by the trailer to a third party whilst being towed. For example if you reversed it into something your car insurance would pay out for the third party's damage. However any damage to the trailer itself wouldn't be covered unless it had its' own policy. If the trailer damaged a third party while it was unhitched (for example if the handbrake came off and it ran away or if you were moving it by hand and bashed into something) you wouldn't be covered without a trailer policy. It isn't a legal requirement to have insurance on the trailer itself though, just a very good idea!