[phunkyfish]

Ok, so the variable I was using on the guide screen seemed to be issue.
I spotted loads of wrong durations earlier today, but then realised I hadn't remembered to change the code to the revised listitem label. When I did, everything worked as expected. Don't ask me why, because I have absolutely no clue why a variable I've had in place for months suddenly stopped working on that particular screen and why it continues to work without issue on the PVR programme Info screen.....Just one of those many Kodi mysteries I guess!
Anyway, I'm certainly not going to complain about having a much cleaner and more efficient approach!    Credit to yourself and Ksooo for the suggestion.
Cheers

You’re very welcome. The credit is all for @ksooo