OK... So I discovered a better method, courtesy of another forum. My original method didn't allow the recorded Midi to trigger the other synths, but this one does. So essentially it's the Holy Grail that allows everything to be done in one Midi track. So any edits affect all the other tracks automatically.
The forum is https://discuss.cakewalk.com/index.php?/topic/19314-one-midi-track-multiple-soft-synths/
and the important post is by rsinger on Sep 7th 2020, which says 'Add your first VSTi with MIDI out enabled and then add a second VSTi instrument and on the MIDI track that is part of the second VSTi set the MIDI Input to your first VSTi.'
So... Track 1 is the first VSTi Track . (Inserted as a Simple Instrument Track, and with Midi Output enabled)
Track 2 was inserted as the second VSTi, but this time with the option changed to Midi Source. And then (as per the instructions) with the Midi input changed to the first VSTi.
Track 3 is the second VSTi inserted again, but this time as a Simple Instrument Track. And the output of Track 2 changed to point at this VSTi.
Happy to answer any questions. (Although I'll not be available much in the next couple of days.)