UPDATE:

Well, we noticed something else today, which I'm annoyed I didn't spot earlier. There is air leaking from the no.1 spark plug hole, how this is happening I'm not sure, but I'm pretty sure this is what is causing the ticking. The plug isn't firing properly, though it does still fire occasionally, so something has happened to it. Can anyone suggest what that might be? I am assuming it has just fallen apart.

The plugs haven't been changed since new, it was assumed they were part of the service schedule, which they are not. So, they are properly rusted into the head. I can take it to bits to investigate damage, but it will have to wait til after xmas now. As for rebuilding it if anything internal is damaged, well that's beyond my skill, not to mention the fact that I might not have all the tools for the job.

My initial fears are that part of the plug has broken away and has ruined the bore/cylinder/valves etc etc. It's a head off job either way, I just hope it's not too badly damaged as my girlfriend can ill afford such an expensive repair.