3 things to check and possibly replace that are often mentioned on here a lot as common faults and are not included in service intervals etc are:
Cam Follower
PVC Valve
and the Diverter Valve.
If you are going to replace them look at replacing with the latest revisions
They are cheap to replace especially if you do it yourself. Easy to do even if its your first time and there is plenty of Guide's / How To's on here if you have a search for them.
Update This is for the GTI's