Motor Insurance with Trailer

I seem to remember a post that an insurance company was obliged to cover you for third party in respect of damage caused by virtue of towing a trailer/caravan as long as it is legally towed under the road traffic acts.
Can anyone confirm this is still the case ? I'm having problems getting such cover with most insurers saying they don't cover it. I'm not sure the people at the end of the line actually understand the issue.

Your motor policy covers you for TP damage, caused by the towed vehicle.