Grinding noise when towing

Sounds like the alko hitch pads are  in need of a good clean or replacing ,

Cleaniing with brake cleaner spray can help 

I take it your hook is also not contaminated as it also has to be clean and free from any paint

Is this the usual heavy creaking noise that's often apparent when manoeuvring at low speed? Difficult to say really.

Jvb66. Will give everything a good Inspection and clean, thanks for the brake cleaner tip.

tinwheeler. That’s exactly how I should have described the noise.. but it has become worse and more frequent almost unbearable on roundabouts. 

I assume you have removed the black paint off the towball when it was new? I always wipe the towball to clean off any rust or debri before each trip. I still get a certain amount of grinding on roundabouts. It's the friction pads on the  hitch designed to reduce the chance of the van snaking.