Check the cover under your motor insurance. All insurance will cover third party damage caused by a trailer when attached to a towing vehicle. Anything more should be checked as not all policies automatically include any further cover.
Check the following:
Does cover mirror that of the towing vehicle?
Is cover provided for trailers whilst detached from the towing vehicle? If so, is it at own premises or specified third party premises?
What cover is provided by subcontracted hauliers when towing your trailers?
Check your exposure when towing third party trailers. Are they expecting you to cover the trailer for damage when in your care? Is this whilst attached and detached?
If the motor insurer cannot extend to the cover you require, it may be possible to add to the transit liability policy.