A frame noise when towing

It might be the friction pads in the stabiliser making the noise. Quite common.

Probably as Freddy suggests .... try cleaning each pad with a little emery paper/cloth and wipe with brake cleaner. Also make sure the tow ball itself is clean.

If it's a clunking rather than a creaking sound and you are hearing it mainly when accelerating or braking I would suspect that the hitch damper is shot.

Thank you again for the comments, I'll get it to the dealer next week. 

Also check the towbar mounting bolts are tight.

I agree we had a similar noise some years ago and it was found ro be the Towbar bolts needed a tweek with the torque wrench

Clunking, especially accelerating or braking, I would suspect the hitch damper. But also badly adjusted brakes will cause the hitch damper to work excessively.

Creaking, especially noticeable with slow maneuvers, would point at the stabiliser pads. 

Maybe, but they should never be so loose as to allow play which would cause a clunking noise.

On ours it sounded louder than the real problem as the vehicle body amplified it, 

Any of the replies could be the cause when was the c/van serviced,come to mind also

Serviced February 2016, travelled about 1800 miles this year. I've checked the bolts all tight and secure. Local Lunar dealer looking at it tomorrow.