Eureka!

When I first load the offending file, the Tick Offset in Notation Window Options (Opt.=>Preferences=>Notation) is set to 0. The first time I re-generated the file the Tick Offset changed to -14, and the triplets no longer appeared correctly.

I tried again, and got a value of -13 - in both instances I'd stopped the song from playing before the end, so there appears to be some dynamic adjustment to the tick offset as the song is played.

Going back to preferences from Chord Sheet view seems to restore the default Tick Offset of 0. [PS: not every time!].

The Style I'm using is _JGTFRFS FastJazz w/ FreeCompGuit (190RS).

Many thanks, Noel, I had already found a way of ensuring that the notation would come out the way I wanted it ("print" it to PDF immediately after opening the file, and before generating the sound-track), but you've given me an understanding of what's going on. I have a tiresome personality trait - learned and nurtured at school (just the day before yesterday...) - I have to know "why?" when I'm told to do something a certain way, not just "what?", and I waste a lot of time gnawing like a dog at a resistant bone until I can work it out.

With your help, I'm now able to get back to playing my guitar again!

Michael