Hi Gaz ,
There's about a 1/4 turn before they seem to lock up put penetrating fluid on them & at the time there was no change. The engineer I use is about an hour away from me so we have decided to leave it for a while & they will come out with the engine as hot as it will get after an hours drive, think that might work?
When you do it Richard, use a long a socket bar as you've got with as short an extension as you get away with and try to keep the socket and extension in line with the plug to avoid any side load which will snap the porcelain. You do not want any of that getting into the cylinders.