The axle gear on my Q1 started slipping after several months use. Since it was within warranty the dealer sorted it. Apparently a recognised happening, randomly afflicting the occasional loco.
Hornby's manufacturing contractor does seem to have some tolerancing issues, both on rotating parts which should have a clearance and components which should be a good interference fit like the axle gear. A well known example of the first was the screaming bearings on 08 shunters. Before these problems were experienced I had been very surprised to find that the brass bearings on driven wheel axles were matched to axles, and could not be freely swapped around. Now that's not something which falls within reasonable use of the product, so not something I would complain about to Hornby; but it does indicate some lack of manufacturing process capability in my opinion.