Pete,
As Iain says, just get the pointer in the right place when you replace it.
The way i did it was to drive the car at a steady 60mph - speed taken from Satnav, NOT dial face.... note what the speedo says (in my case I can't remember, so lets assume it was 65mph???).
Stop car (obviously!) and remove instrument cluster.... You'll have fun with that bit

- it comes out the top of the dash and is fiddly rather than hard to orientate the proper way to pull it out.
Dissmantle the speedo so that you can get to the needle and then the face, but be able to view the rear of the mechanism.... hold needle in the 65mph possition (or whatever your number was showing at an accurate 60mph), make a mark against the rear rotating disc (that the needle attaches to), and an adjacent mark on the body of the speedo (think 'Timing Marks' on the engine).
Now you can remove the needle and the face, replace with new face, and replace needle. Simply rotate the rear disc until your lines match up, then slip the needle on to point at 60mph. (or in your case, for your new guages, 98kph

).
reassemble and drive - you should see that the speedo is now accurate at 60mph when compared to SatNav (you may want to do it again to get it even more accurate if it didn't work well enough the first time, so just try it before you fully reassemble the whole lot).
I did mine about 6 years ago and it is absolutely spot on at 60, and within a couple of mph everywhere between 20 and 120.
Not a hard job, but required some dexterity and patience.