Ha, you are a clever lot.
We've already tried epoxy with a 'wrap' of thin aluminium sheet. Lasted about 30 seconds.
The 'drill and pin' idea is a good one, but I think the arm is too thin for it to work. Either the pin would be too small to hold, or the hole would be so large that the arm would break (fatigue?) again. So my solution has been to ask a friend's husband (he's a professional metal worker) to make up an iron sleeve to fit over the mounting arm. This will support the thinnest part of the mounting arm and I can then pass a bolt up vertically through the sleeve into the existing threaded hole underneath the centre section of the model. I think that should do it. It ought to be strong enough to reinforce the arm, which we know is susceptible to fatigue, while not being too unsightly from above and holding the aircraft in the right place relative to the ashtray. I suppose we should call this mounting the Mk IV...
The most interesting thing about this model, though, and I wondered whether anyone can tell me anything about it, is that it does look rather 'handmade' and I wonder how many were made originally? The wings have no aerodynamic section - they are more or less flat - yet the area around the engines looks definitely hand-tooled.
I always assumed it was a promotional gift from de Havilland's very successful sales campaign for the Comet 1 and 2, but perhaps it was a one-off made by someone who worked at de Havillands and mounted on an off-the-shelf ashtray?