Solved!
So I managed to fix the issue. It turns out it was an audio driver issue after all, only one masquerading as a Pro Tools error.
I was running Scarlett MixControl version 1.8 (which is the latest version according to the Focusrite site). I actually tried re-installing this same download 3 times only to end up with 3 different results - one of them being the situation detailed above, the other being crashes when using the video engine and the last being where Pro Tools simply refused to open a session without crashing.
After calling up Focusrite again, they directed me to a beta release of a newer version for the 18i8 (specifically for Windows 10). If you follow
this link and scroll down until you find version 1.9b4, install that and hopefully your issues will be no more - it worked for me.
Hope this helps anyone suffering the same headaches as I
