Thank you so much for your help. Sorry about typos leaving out $ signs etc, although actually I think I got them right when experimenting.
Of course your solution worked perfectly, so I don't know how came to believe I couldn't use functions in Actions -- I must have done something silly, and then been too discouraged to continue because I'd already found that if I try to introduce a function in the Action: Replace, then it seems to replace with the function description I entered instead of the function result I want. Is that what is meant to happen? And yes, I have Track numbers OK.
But anyway, when I tried functions in Format value as in your suggestion it worked (so goodness knows what I did wrong previously), and thanks again for your patience in providing such a full explanation.
I wanted the movement numbers in the titles just to show which movement out of a number of movements was playing, but if I could have programmed in multiple substitutes of I, II, III, IV for 01, 02, 03, that would be even better. Is that sort of thing possible with this arcane Regular Expressions stuff?
For some reason I have a particular problem with German software, in that I sometimes get the feeling that help files are written by people who utterly understand how their system works, but have forgotten what it's like to be a keen but clueless first time user. In the case of MP3TAG, they provide a perfect single representative example of each feature, but only one example per feature -- after all, why clutter up the help pages with more? Well, a bit of extra clutter in the form of 3 or 4 varied examples of the same feature would help a moron like me begin to discern patterns of use that reveal the underlying principles that the experts understand so well that they don't think there's anything TO understand... But then, the best German software is just so much better than the competition that a steep learning curve is worth following. For me and my colleagues, Samplitude, for instance, has been just miles ahead of all other audio applications for at least 6 years.
Anyway, sorry to rabbit on, and thanks again.