Question Author
For the benefit of anyone with a similar question who comes across this thread looking for answers, it all went smoothly when I took action a few days ago, and what I ended up doing was:-
- created new gmail account and added that to the phone.
- switched off the phone (very probably unnecessary but I did it anyway)
- deleted old gmail account using my PC
- turned phone on and when it prompted for gmail log-in details simply removed the old account, causing it to automatically use the new one as default
The only app that then complained about the lack of gmail account was the calendar which synched with it, so simply set that to use the new one. It would have been a good idea to have moved events from the old google calendar to the new one (I use the Lightning add-on in Thunderbird so this action would have been simple) to stop them from being deleted along with the rest of the account but I forgot to do that even though I had thought about it beforehand.