During my cars SVA test the Speedo results showed that The car would be doing 72 mph at 70 mph and 41mph at 40 mph.
If the calibrating factor is - 42.5 at the moment (Hopefully I just read it right ,its been awhile since set it up) which way should I adjust the correction factor and by how much to get it under the speeds?

Online Speedo healer calculator
http://www.speedohealer.com/eng/adjust.htm

Should help you, click on precise ( actual speed and indicated) fill in the values as in the table click calculate, and it will give you the error factor, if you click generate instructions it will tell you how to programme it.

Mike