Hi I have an audi 100 which is fitted with the bog standard 15" wheels. I'm currently about ot buy some S4 alloys 17" for it, I've looked the PCD and everything is the same as mine now, so I'm 100% sure they'd fit or am i?

Anyway apprantly if your car has 15" or some size alloy and you get bigger ones, you need to take it ot a garage and get them to sort the speedo, is this true how much does it cost roughly?

 

You only need to recalibrate the speedo if the diameter of the tire is much bigger or smaller then what the original design called for. Tire makers provide tire data on their web sites. There is a formula for calculating the correct aspect ratio so that you don't have to recalibrate anything, it’s also called plus sizing.
ex. 195/60R15 = 215/50R16 = 235/40R17 should have the same or similar overall diameter. However I don't find it too accurate and sometimes the width you get might not necessarily fit and clear things under the fenders or the size is not very popular or common.

Approximate Diameters for:
195/60R15 = 24.2”
215/50R16 = 25.2”
235/40R17 = 24.4”
205/45R17 = 24.2”

0.5” inch is not going to make a major difference in speed and will add or deduct about 1MPH@60MPH on your speedometer reading. Sometimes car manufacturers will actually overrate the speedometers so as to compensate for tire manufacturer variations and avoid any possible litigation.