Trailer update: I've now been using the new Bellelli trailer for three weeks and there have no problems with any of the three bikes I use it on. I also compared the tongue weight of the new Bellelli trailer vs my old Halfords trailer and the difference is huge – the back end of the bike feels really heavy with the Halfords trailer attached but light as a feather with the Bellelli trailer attached. This, despite the Bellelli actually being about twice as heavy (in gross weight terms), due to a much more robust construction.
So, it seems as though the usual law applies – buy the best you can afford cos you get what you pay for.
Thanks for all the tips and advice. Seem to have got to the bottom of it, as far as I'm concerned anyway! Hopefully this thread might help someone else with the same questions in the future.
Ride on!