Rake Going from (suggested) 43mm to 40mm
 

So on the site it says that the rake of there fork is 43 but seeing as I have to buy a seperate fork I was looking at one with 40 mm, will the difference mean anything other then the bike being more twitchy (will it fit, actually work with the frame) the frame is a leader 730tt.

Sorry for all these questions I am building my first bike and am a noob when it comes to things like this.

Reducing the rake offset from 43mm to 40mm will increase steering trail and slow the steering a bit. This is exactly what I did to my 5500 Trek.
Adding rake offset reduces trail, and that can make the bike feel more twitchy, you're talking about going the opposite direction. Here are some links to help you:

http://traylorfwd.home.mindspring.com/geometry_def.html

http://www.kreuzotter.de/english/elenk.htm

Al

+1 I'm glad you posted the proper description as there seem to be so many riders who can't get the idea straight of how rake and train are related.