i've just got my self a 40t sprocket of fleabay for £15 delivered.
will i need to make any changes to my speedo, i've heard of other bikes needing a speedo healer when changing gearing?

Shouldn't mate, the sensor is a magnetic pickup off the front wheel.
The 17ws you se when you start the bike tells you that it's configured for a 17 inch front wheel

thought that was the case but wanted to check to be sure.
cheers.