+1 for the Plusgas approach, but make sure you find some Plusgas - local motor factor or metalwork supplies outlet is your best bet. You need a dedicated penetrating fluid and despite what is says on the tin, WD40 is not as good. As Ian says, do it over a few days to give it time to work and go easy. If need be, tip the bike on its side so the Plusgas pools around the bottom of the plug hole and do them one at a time. Its still going to work out quicker and cheaper than new heads! Lump hammers are the absolute last resort and should be thrown in the same bin as adjustable spanners.