Thanks a heap Hadron! I did what you suggested, and I'm nearly there. Step by step:
Found and ran the official Gingerbread RUU.
Ran revolutionary for s-off.
Flashed Mildwild 4.3 (Oxygen).
But still no sd-ext. Doesn't seem to be recognised by MildWild or any of my apps. I formatted it again through the computer, and also did a factory reset which said that it formatted sd-ext. Any ideas for how I can get the sd-ext to work so I don't run out of space again?!?