Ok fellas....I'm damned if I know how I got that .exe. file...but anyway, as OF suggested, I started again!
So....I downloaded that .iso file of Eric's, burned the disc and ran it.
Well, considering most of the stuff on it was gibberish to me, I gave it my best shot! It doesn't appear to have been a complete disaster, far from it. I apparently have managed to format that unused partition to ext4, the other partition is the same, and I can see it in GParted.
But...I can't see it in the boot list when I start the machine, and don't know how to use it!