So, I got it running!
And rather than take it to someone that knows what they're doing, I decided to work it out myself in an effort to becoming one of those someones.
Basically, I worked out that there are 2 main places you can shim a starter motor. You can shim it between the starter and the block, or you can shim it between the bolt heads and the starter. I was shimming it between the starter and the block, and it just never made enough of a difference. As I was pulling the starter off to try something else, I dropped one of my makeshift shims into the bellhousing. So, I pulled the bellhousing off (realising then that I should've done that before I did anything else, right at the start!). Then I put the starter on with the bolts and no shims, and looked at how it sat when positioned normally. It appeared to be ok, so I decided to bolt it up properly and then watch it run with the bellhousing off. What I found was that it was jumping around on the bolts. It turns out that the bolts would be tightly screwed in, but because this starter motor is a bit more shallow than my other one, the starter still had a tiny bit of play. So, I put shims between the bolt heads and the starter motor, and bolted it in. No play, so that was a good start. I then went to start it, and it was misbehaving. I figured it was a battery issue (ran it down too far, trying to start the thing all the time), even though the volt meter said it was ok, I decided I should charge the battery up anyway. So, after charging, dumped it in, turned the key, and it roared to life!
So there you go, something I wouldn't have even thought could be an issue, but now that I've fixed it I feel more educated, ready to tackle the next problem the car throws at me :)