Your test worked as you described (on a single file) on my laptop.

I wonder if my mass retagging in iTunes needed a while to update all the files on the iMac and therefore the sync didn't pick up the changes?

I will have another go on the iMac later today to see if it works.

Otherwise, I think I will have a longer look at Tagr.
http://www.entwicklungsfreu.de/tagr.jsp

Thanks again. (will try to remember to report back!)

Video Sources: Cheap Samsung BluRay, Xbox One, Virgin TiVo HD
Audio source: Hifiberry DIGI on Raspberry pi running
max2play.
Music server: MacMini '06 running iTunes library
Output (3.0): G61R, HDmax421, DSP5000, DSP33C and Samsung 29in LCD