Managed to get it in. Long story short, I don't think it's the screws, it's the holes. On closer inspection they seem to be just that, plastic holes. So they'll take varying thread types put you have to put a bit more effort in to cut into the plastic. Apple's slightly more aggressive threading and the slightly odd cross head slot made it hard to apply enough force. They probably would work but I felt a bit nervous about having to forcing a screw into a brand new SSD!
I used hard disk mounting screws from the Antec P180 case our PC is in which are longer on account of the vibration dampers in that case. Will upload a picture of the 3 screws nect to each other tomorrow so you can see what I mean.
Thanks for help and suggestions. The SSD must work as its now running the Lion I'm typing this from 😎. Boot up time noticeably increased, haven't had much chance to check anything else yet.