So I’ve (kind of) fixed it.
I tried re-installing CP8 Pro on my Surface Pro 3, but still the same problem.
I tried removing all VST paths from Plugin Manager, except the default Cubase ones.
Still no joy.
So, then I installed CP8 on my Asus Vivotab Note 8, copied the project folder to a USB stick and copied it to the Asus.
The project opened fine on the Asus, but gave a notice about missing plugins, which was to be expected.
I saved the project with a new name and then copied it back to my Surface Pro 3.
I put all of my plugin paths back and it opened up the new project OK. All plugins working as they were the last time I saved it about 12 hours earlier on the Surface Pro 3.
Not sure why the project file would open OK on the Asus Vivotab Note 8 and note the Surface 3.
Anyway, If anyone else has a similar problem, maybe try copying your project folder to another machine, opening it and saving as a different filename and copying it back.
Not sure why that worked, or exactly what fixed it. But, I’ve got my project back, thank goodness.