W reg 1.4 Ford Fiesta?

New to fuel injection engines.
I have a misfiring engine on my fuel injection car this is the first one [used to working on plugs, leads points and distributors. My that's going back abit! ]

We paid out 4months ago over £700 to have new head gasket and garage said the head has to be skimmed as it had distorted the head driving when it was very hot to get home from 1/4 mile away but really boiled at home.
I need knowledge amunition before I contact my garage as to what could be the possible cause. only one cylinder not firing up.
my own assumtion with my little knowledge on these engines is:
Dirty injector?
Work injector?
Easy to change ? is it a DIY job?
Or what?

any suggestion would be gratefully received, apart from sell it or scrap it.