Tried out some of your suggestions. Can't seem to turn Opcache off without switching it off entirely, as it's coming through php-fpm. I guess that's understandable.
I've found 2 work rounds which may help others:
This throws the current file out of the cache. So obviously I can either put it into code I'm changing right now, or set my app up to do this in the common include when for example it sees me using my test domain.
The other quick fix is to change the re-validate frequency to a lower value. Default is 60 seconds.
As a professional developer, you are absolutely right, don't put code on your production server that you're not sure of. But when a customer tell you, that either a) it's already broken, or b) there's a daft typo, then being able to quickly fix without the 60 second wait between uploading a file and getting the result is of benefit.