Daniel - have sent you a PM
AJ Stars - I'm not sure if it is the damper at fault or just poorly set up brakes on the trailer. Lately I've noticed the trailer jolts the car when pulling away after coming to a stop.
The dampers are easy to change and cheap - £43 for the correct one for this hitch from Ebay
KNOTT AVONRIDE TRAILER DAMPER 87001109 1300KG - 2000kg - IFOR WILLIAMS P00373 | eBay
I've just replaced everything on the trailer with new - complete hitch assembly, brake cable for threaded bar and brakes. Don't know what was at fault didn't really bother to look too closely. The hitch is fine as far as I'm aware - I was mostly concerned I only had 1 key for it.