Export from Sibelius without keyswitches. That low note is almost certainly a keyswitch.
If you export as XML rather than MIDI, you should not have this problem.
It is imported via mxl… (compressed file… - not xml…)
I did another import - actually two - both xml and mxl. And this time I get no low note.
Must have been an import or export glitch…
All is well now
I expect the problem was in fact caused by having the wrong VST expression map chosen in the Endpoint Setup dialog for the channel playing the viola: if there’s a mismatch there, then a note that is supposed to produce a change of playing technique but which is not mapped to a keyswitch in the patch that’s actually being used will produce a low pedal note instead. Changing the VST expression map in Endpoint Setup is the way to solve this.