Stuck Spark Plug on renault K4J 1.4 engine

Hi, I went to replace spark plugs recently (first time during my ownership, car at 36000 miles) and I noticed as I worked through the plugs from left to right (car is an 05 megane II hatch) there seemed to be more and more corrosion in and around each chamber. the 3rd plug came out with some struggle, but there was too much corrosion around the 4th spark plug chamber, I stopped trying before I ended up completely rounding the head off the plug, and decided to leave it to a mechanic. My local friendly mechanic, who's very trustworthy, and an excellent chap all around (godsend given my previous experience with mechanics) didn't want to touch it, as he said it could get very expensive if it goes wrong, i.e 800-900 quid if the head has to come off etc because that would mean timing belt etc etc, shame as I've literally only just had timing belt/water pump/etc done.

Are there any tricks to getting a spark plug that's stuck out? and how can I remove any existing corrosion around the top of the head and in the spark plug chambers to halt the corrosion in its tracks and stop it spreading further? I replaced the four ignition pencil coils sitting in the chambers on top of the plugs as I think at least some of them looked badly burnt from whatever corrosion had been happening in and around the last plug or two.

any advice really appreciated. I'm based near Luton, so if theres a garage anyone knows of that specifically specialises in this kind of work (maybe its just something my mechanic doesn't want to tackle as he's not extensively experienced at it?) a reccommendation of such would be appreciated

thanks!

thanks lots guys,

pumaracing's technique (hot engine, downward shock tap applied to metal body of plug, avoiding insulator) to help crack combined with some plusgas, which definitely helped, and really does seem to "dissolve" rusty bits into brown mushy liquid quickly, worked a treat.

my main concern was , and is, what effect the bits of filth that will have fallen into the hole in the head for the spark plug after spark plug removal will have - is this dirt/few millilitres of plusgas falling into the engine via the spark plug hole likely to cause any long term damage? the engine did start ok thankfully, and is running much smoother.

I can think of no good way to properly clean out all the corrosion and dirt from the recessed chambers in the head which hold the pencil coils and spark plugs, can this only be tackled with the head removed?