I have received the faulty bogie that Roachie sent me, and examined it carefully.
There are two problems here, first of all the gear muff is split on the short side, this means it does not hold the wheel and stub axle firmly, so there is a possibility of slipping. This problem could be alleviated by carefully cleaning the gear and stub axle in alcohol, then placing a tiny, and I mean tiny, drop of Loctite 638 or 609 on the inside of the muff and pushing the axle back it place, using a back to back gauge to make sure the wheels are in gauge.
It could also be fixed by Trainorama supplying replacement gear muffs, but that is something to take up with TOR.
The second problem is probably more serious and prejudicial to long term reliability of the drive train. The intermediate or idler gears in the power train are very narrow, in my opinion, far too narrow, and there is no reason for them to be so. If they had been designed properly, they could have taken up almost the entire width of the inside of the plastic gear casing. The spacing in the gear casing is approximately 5mm, and yet the idler gears are only approximately 1.4mm wide. The axle gear is 2mm wide.
The root of the problem is that the idler gears are able to slide sideways by a little greater than 1mm, meaning that at times, less than 0.4mm of gears may be in mesh. Under load, the gears are able to force themselves sideways a fraction more and suddenly one tooth jumps over the top of the tooth it is meshing with on the next gear. If this process continues for any period of time, the edges of the teeth will round off and suddenly you have no drive at all. The rather strange gear tooth profile also compounds this problem.
This would also account for why Captain Underdacks & Roachie noted that the clicking only seems to occur on grades or under load, not on the flat.
As a result, I am surmising that the clicking sound heard by Roachie and others is due to the gears jumping teeth, as Roachie suggested earlier, and there is no simple fix for this. It would be possible to replace the gears with metal gears, but the cost of making those gears combined with the labour costs in disassembling the model and replacing the gears would make the exercise uneconomical.
I am at present trying to coax my camera into behaving properly in the macro mode so I can take a photo of the gears to illustrate the problem. I will post more when I have done so.