Thought I would return with my feedback as it may help others starting out (probably not experienced folk) and changes were relatively simple. Another GTV learning experience for me to a list that is getting longer and it was satisfying (for now).
Anyway, I had the car up on stands yesterday so I could get a good look at what was going on when the car was started and just for access more than anything. I also had the mounts off. Good news, both were actually fine. As I had bought a powerflex bush for the top engine mount I replaced it. Not so good news, when going to remove the rear mount, whoever checked/had it off previously had not secured it - one of the bolts had just been screwed in by hand half way.
With a good friend helping throughout (BMW Youtuber Elbow Grease), we had a look and it was quite obvious there was too much movement in the mid-section of the exhaust including the CAT. There was a bush in the middle of the car, nearside that was perished and causing a lot of movement. Some more of the exhaust bushes were perished. Several exhaust bushes/hangers (some creative because of availability) later, mounting points off and cleaned, car started, the difference was noticeable particularly when idling, when up to temperature and applying throttle. Everything secure meant that the Flexi's were not all over the place especially when applying throttle.
I went out for some low gear, high rev GTV/Busso fun and so far, so good and no signs of early squirming or buzzing that had been common from the off before. Even better, I went over Hampton Court Bridge and in traffic, I was approached by a couple through the window who told me that I have a very nice car. I may just get a stress free Sunday drive out of her today as well!
Still having paid a garage £300-£400 to sort out the exhaust, quite disappointed that this was not diagnosed, seen or even advised.