Yes it seems there are lots of different spellings for lots of words…just to keep you on your toes!
I did the southern course - like you I think … at the time it seemed to map pretty well onto what Iestyn was saying - as a rule of thumb anyway.
He said io… on heibio …and an ‘o’ on parcio…didn’t he? The trouble is that seems a long time ago now though and I’ve been listening to a lot of Northern things recently …so the audio recording in my head is not as clear as it used to be!
However if rules always have exceptions, I’m not sure what things that aren’t quite rules have!!!
It’s a good excuse to say it however you darn well please to my mind - it doesn’t really seem that anyone can say it should be one way or the other!!