I found a film canister that I managed to just squeeze into the focuser, so I gave collimation a go following ballaratdragon's instructions. At first it looked like I stuffed it up so bad that I'll never be able to fix it; loosened the screws holding the secondary too much. After regaining my composure, I managed to secure the mirror again and a 1/2 hour of fiddling later, it's as good as I'm gonna get. I really wasn't game to take the primary out for "spotting", so I used a laser pointer blutacked to the inside of the tube to shine on the centre of the primary.
All the screws on this thing are badly corroded, which makes adjustments all the more difficult. They are just covered in what looks to me like zinc oxide, and I think a fair bit of that powder ended up on the main mirror.
After all this the image quality is not really any better. With the 30mm EP everything looks nice and clear. With the 10 & 6mm it's hazy, even in daylight. These smaller EPs did look pretty dirty, so I attempted to clean them, dried them with a hair dryer on low heat, and then they fogged up on the inside.
Now I'm waiting for them to dry.