Good news everyone!
I've ditched Final Draft 7 and gone forwards to Final Draft 8, and huzah - I'm back in the script writing saddle. It feels good to finally be able to sit down without worry about that weird glitchy software problem I was having. A re-install of FD7 would have probably worked, but if it happened once, what was to stop it from happening a second time?
So therefore - Final Draft 8 - the only downside I've found to it though, is the slightly more circuitous route you have to take to save as a PDF. Rather than, as it was before, simply going File / Save As PDF ... you now have to go File / Print / then down to the bottom of that pop up window and select Include Title Page / Print to PDF ... at which point you can save it as a much-needed PDF.
Utterly bizarre, but as I can still save to PDFs - the most important thing - then I'm fine with it. Care-free script writing is what I want and I've certainly been enjoying it, as I've finally been able to sit down and power out the first pass of the new opening scene to "Zero". It's come out nicely, but I'll be looking to go back and add a couple more pages (I used five, but my limit for that scene is seven) and generally tidy it up a little bit.
Naturally I've had a few other thoughts of tiny tweaks or the odd line of dialogue I can add in, so I'll be skimming through the entire script once again to really make sure it's spiffing before sending it on to the BBC Writersroom.