I am considering this too, as the wheels, for me, are not close enough to look flush with the lines of the car. Yes. They are a SAXO owners wet dream and I completely agree with most of the comments that these cars should be left alone. I also wish to lower the car by 10mm , again, as I think the ride height is a touch too high.
Get your car on a flat piece of ground, level too. Get a piece of string or a plumb bob and hold the string from the inner lip of the wheel arch (To ensure you don't risk rubbing the arch when you bring the wheel out) then measure the distance from the string to the wheel (maybe 10mm.) This is the size spacer you need to get. Check the different types, you may need a hubcentric spacer with it's own threads. Check the thread measurement, I believe they are M.12 x 1.5. Check the extending thread currently on the hub assembly doesn't foul the wheel and stop the spacers from connecting with the wheel correctly.
All going well, fit the wheel and enjoy a slightly wider track that will have a minimal improvement on the handling as you will be 2cm wider now, watch out for curbs and contact your insurance provider. There will be a change. not much, but best to notify them encase something happens, related or unrelated, they will find a way to get out of paying if they can.