Wow, it was a plugin that was causing the problem. I’m really confused as the problem happened the same time as I removed a theme file which doesn’t really make any sense in my head but not to worry, I’ve reactivated the only themes I actually use and all appears to be fine.
Have lost all my customisations after having to forcibly reinstall Atahualpa during my earlier troubleshooting but that can wait until the morning (not sure where you are but it’s midnight in the UK).
Thanks again for your help; it’s really appreciated.