Thanks for your suggestions, Richard.
I recently purchased a lathe on Ebay (under £30 and not very sophisticated, but it does the job) and I experimented the other evening with reducing the length of an axle. (Initially I used a regular file, but found it difficult to produce a pinpoint end to the axle – presumably because this part of the axle has barely any movement when rotating in a lathe. I solved this by using a disc on my Dremmel as the file, with the lathe simply there to ensure the filed axle remains perfectly symmetrical.)
I think I can do away with the metal bars completely. I purchased some wasted pinpoint bearings, which fit neatly into the axle hole on the plastic bogies once the metal bars are removed. The issue has been that with these in place, a standard 26mm axle is too long. So I think your suggestion of reducing axle length is the answer.
With the wheels stretched out to EM gauge, there’s just about enough room to accommodate them within the bogies without the need for filing (as long as the axle cups hold them precisely) so I may be able to get away without filing the bogies, or only filing a small amount.
I’ll start experimenting next week and see how I get on.