Hi Steve, I used to work for a company who built trailers and we always specified Avonride axles as we found them the most reliable. When RM Trailers were building the road trailer for my P235 I paid extra for them to fit Avonride axles, as opposed to the ones they were using.
So it all sounds a bit fishy and probably was a case of the wheel studs or nuts not being tightened properly. Long gone are the days when they used an impact driver, which meant it was difficult to change a wheel. Now legally the studs have to be tightened with a torque wrench to a specific figure.
So if the mechanic was able to fit the spare wheel, presumably the studs were still in the hub. Did he bring some more wheel nuts, as presumably again if the wheel had come off, so must the nuts? If the hub had failed, then he wouldn't have been able to fit the spare wheel. As to whether the hub is wrong for the size of the wheels, again presumably the number and pitch of the studs and wheel must match. If the hubs are too small for the weight carried on the trailer, then the bearings might overheat and you would have the same problems as Ken has had. However this is normally because the hubs have been immersed in sea water and not serviced. If the hub failed, resulting in the wheel detaching, then you certainly wouldn't be able to fit the spare!
The use of the torque wrench can have its drawbacks. On collecting my Citroen after a service by the Dealer, I immediately stopped and returned due to the noise from the wheels, and it was a case of loose wheel nuts. The explanation was that the apprentice put the wheels on and then the Technician was to finally tighten them with the torque wrench. However he was called away to a staff meeting and afterwards it was the end of his shift and he forgot to check the wheels. Fortunately I was alert enough to stop when I heard the noise, but doubtful if you could hear this on the trailer.
Peter