Had my lesson this evening, seems to be combination of things.
First thing I did was hand my teacher my horn and he commented that it's "a brilliant tenor" but he wasn't keen on the mouthpiece.
Naturally he enquired why, and I explained that I was still struggling with upper register (particularly A) he played again and had no complaints beyond the mouthpiece, but highlighted it proved regardless of set up he still sounded the same.
He understood the change to a 2.5 reed, but I still having issues, so lots of experimenting whilst he studied what was happening.
I need to drop my jaw a little, and work on my embochure We tried a different mouthpiece, and I still had issues so it's most certainly me.
We spent a bit of time working in these on my original mouthpiece, including overtones, some weren't very forthcoming, and he suspects the mouthpiece might be too open.
It's a refaced Link 6* (.100"), I have a T6L (also .100") that I leant to friend so i'll give a go of and see if that's better, failing that I'll be on the look out for a narrower tip piece.