After 3 hours of strategising and swearing, I got the offending piece of plug out of the combustion chamber on Sunday - phew!
In case this happens to anyone else, you might find the following useful:
1. The blu-tak idea suggested by pkr is definitely the best place to start. It would have worked great except that the piece to be pulled out was wider than the hole. So, the blu-tak on wire grabbed hold of it but could get it through the plug hole.
2. I got it out by hoover. You need to put a piece of plastic tubing on the end of the extension and slice it with a knife at an angle so the the opening is as wise as possible.
3. Put in the other spark plug on the cylinder as the vacuum created increases the suction power of the hoover.
4. You need to raise the piston but not to maximum height as this restricts access and movement of the hoover tube.
5. I could not find a socket to turn the cambelt from the bottom pulley. It seems to be somewhere between 18mm and 19mm. Instead, I turned it clockwise from inside the cambelt cover on the cog nearest the front of the car. So taking the wheel and the inner guard may have been a waste of time but it served as a good check seeing where I was compared to TDC marking.
Overall, what a complete waste of time caused by me not being careful enough when I took out the plug. Still, I consider myself lucky that I didn't have to take off the head.