Just a quick one is changing the coil pack a easy or hard job to do or will I need a mechanic 

What car?  Coil packs are often on top of the plug these days and easy to change. 

The coil pack is individual on the ST, so I'd mark up which electrical plug came from which coil before removing if you're in any doubts.

In comparison to other coil pack changes I've done, the ST is a bit more difficult but nothing most people with a decent socket set couldn't handle. Taking the intake pipe off to be able to remove the engine cover is the most difficult bit really, the rest is as straight forward as it comes.

You just need to make sure you have some Torx bits to remove the plastics on top of the engine.

Should be straight forward providing you have the tools. Just remember firing order when putting leads back on. careful you dont drop any Torx screws as restricted working area.