Plugs on a 6n 16v

[99]

Hello

The car's been stuttering on the odd occasion lately, so I figured I'd start by checking the plugs...

My plug socket just to say fits down the tunnel in the head (I could do with a thinner walled one really), but manages to locate on the plug head. The problem comes when trying to undo the plug - I'm having to use way too much force to get it to turn.

Can anyone suggest possible causes? It's all nice and clean around the plug, so I'm hoping no corrosion and I'm using a 14mm socket (if this is the wrong size, that would explain a lot...).

Any ideas would be massively appreciated.

Cheers

Andrew