Ford KA misfires

There are 3 parts of the spark on your KA. Plugs, leads and coil pack. Sounds that your coil pack is the next to change as the primary parts have been done. Does it misfire under load i.e in a high gear when you really could be in a lower gear? Imagine a candle and it gets too much wax and flickers and when you reduce the wax it brightens.
Dont buy a coil pack yet I have a new one boxed surplus to requirements My ebay id is SEAN5545. send me a message and will quote you. This will be the next course if the symptoms are as I have explained.
If you have run the car near out of fuel, you may have a dirty injector. Get a bottle of injector cleaner and add on 1/4 of a tank of petrol. Give it a good run at high revs and it may clear the debris.

If ,as you say , it has only happened since they fitted new plugs and air filter , take it back to Kwik fit and get them to check it foc.Could be something simple like loose plug lead or maybe an air/ vacuum pipe knocked of when air filter changed .