So finally got fed up with the trailer giving the car a thump up the .....towbar when I brake. Formed the opinion that the chief suspect must be the coupling damper (is that the right name?). So I stripped down the coupling to find a little rubber ring dangling on the damper tube, which did not seem right. The ring can be seen in the first picture. It had got a little mashed during its time hanging around. So I visited the local trailer specialists who were able to supply another of these little ring bump stops, and also checked out the damper for me. They said the damper was OK.
So I re-assembled the coupling as you can see in the picture sequence. I adjusted the brakes and went for the test drive full of confidence. I braked for the first corner and "thump", another whack up the towbar. I don't think that I am unduly sensitive but this is making towing a pain.
The brakes were adjusted right up, so that a short trip around the block left the hubs quite hot. Oh, and I get a thump not only when I brake, but as I pull away there is also a thump, the severity depending on how hard I accelerate. I think that despite what I have been told the damper is shot. Can anyone who has more experience than me of these things pass an opinion?
Oh, and big thanks to Trailer Guy for his description of how to get to the damper - wonderfully helpful. Cheers mate