intermittant starting problems on Mondeo TDDI Ghia 2001?

I have a mondeo TDDi nov 2001 Ghia. Recently it has been having starting problems, but not every time.
When it doesn't start, sometimes it just turns over and refuses to start. But when i leave it for 10 or so minutes, sometimes it will start straight away, sometime it wont.
Sometimes when i take the key out and put it back in it will start, but sometimes it won't, there does not seem to be any pattern. I thought it might be a problem with the immobiliser, or the key?
Any ideas.
Many thanks