So first up: thanks for your help!
I did it, but not in the way I was trying initially. I ended up looking at a fairly random looking guide for using the ICSP headers instead of the normal digi IO pins (http://letsmakerobots.com/content/make-arduino-isp-programmer). I don't know if it was due to some dodgy wiring on my part (I think it was ok) or if something was different in using the ICSP headers, but it worked. I DID need to use the cap on the reset pin though, so you definitely helped by insisting I do that.
Happy Leo owner!