ive got a mk2 ford focus 1.6lx core plug 55 plate?

hi ive got a 55 plate focus 1.6 and in the last 12 months its been in the garage twice cuz of miss firing problems which the garage put down to core plugs.im looking at this site and it seems i might have a washer jet problem.but the garage i took the car too sed its not.and it might be that i might have water leaking cause of a head problem even though there is no sign .but all the syptoms that ive looked at seem to point to water jet problems are they easy to fit myself . cheers