Thank you everyone for your answers and sharing your experience, it has been difficult for me to answer before today. It would have been sensible to take some pictures before I posted but as I don't have immediate access to the railway its probably too late.
All the Locos and rolling stock have the (I assume) standard LGB couplings, most have double some single all the freight wagons have single. All freight wagons had plastic wheels, most very worn, all have now been replaced with metal wheels. The track is exclusively LGB and all end connections are now metal screw types. The track is floating with gravel as ballast, the smallest chip size we can obtain is a bit too large but seems to work OK. The minimum radius is 3 and there are no S turns.
In Jimmy's post No 2 he said check "back to back (40 mm)" I'm not sure what that means?
We cannot run very long trains as we are limited by two of four reverse loop sections, I'm not sure what Greg means by "stringlining" but I assume it relates to longer train lengths dragging the front coaches?
There are several posts offering good ideas regarding the freedom of movement of the wheels, correct installation, lubrication, all of which I will follow up and check. The idea that the hooks can be displaced by having a spring retaining a memory after storage could easily apply here and something I would not have thought of.
it does seem correctly laid track is most likely to be the cause of our problem. Greg's digital level and Malcolm's spirit level wagon are good ideas, thanks for the pictures.
We have one spot which causes derailments on a regular basis and where some carriages flip over. This is at the end of the up/down ramp to the garage storage area, it is at the bottom of a straight track with a down hill gradient followed by a 90 degree bend which has a partial gradient transitioning to level. From what has been said above I guess this is a sure fire way of causing problems if the track is not perfectly laid.
Several people mentioned speed as a factor I think this is also relevant to our operation, I will have to tell my friend the owner to slow down.
Robin