Ford Fiesta Finesse 2003 issues coil/spark plugs?

I am a total no-expert on cars, have a Ford Fiesta Finesse semi-automatic 2003 (1388 cc). After I bought it the engine gave up ('spur in the 2nd coil') - replaced at garage, £220. This was in December last year and they said they had a rough look at it on my request and said there was nothing wrong as far as they could see. Still felt engine has some difficulty with getting into gear (speeding up, especially after slowing down and not getting into proper gear and indeed with a nice-and-easy-driving style). Figured out it might be good to get it serviced, especially because of the sensation of another 'misfire' somewhere.
Serviced: new oil filter, air filter, pollen filter and new anti freeze al together £185. Happy all is checked and ok.
Now they found 'core plugs leaking in cylinder head, filling spark plug tubes - require replacing. Spark plugs supposed to cost £20... need two, total cost of replacement: £90! Are this reasonable prices? Thanks heaps