I've just had my first service done and they have changed the oil and oil filter.  When is the air filter changed on a car these days?  I've read that in general they need doing every 15,000 miles at least so it won't have one changed until its on 20,000.  That's assuming Skoda change it then.

 

Does anyone know when it should be changed and if there is any benefit on me changing it before the next service?  Would a drop in K&N or Revo filter make any difference?

You might find that it can be as high as 40k before it's changed. It won't do any harm changing it earlier and depending on your engine it might make little if any difference fitting an aftermarket option.