Thanks for your reply, I thought that would be the case.
They have metal cones. It looks like a solid lump of aluminium with a cone drilled into the middle of it. Surrounding this cone and inset into the metal block is the voice coil. From my research, the SL6's have a similar machine drilled aluminium cone.
Having done a bit more reading, my other option would be to take the rest of the original speakers apart and sell the parts as spares to others in my predicament! Perhaps put the money towards a new set of bookshelves
Edit: I can add some links now:
Celestion 300 (the most information I've been able to find on them is here:
http://www.stereophile.com/standloudspeakers/692cel/ - that link is for the Celestion 100's but as far as i can tell, the tweeters, and cones are identical, the 300's are just taller as they are floorstanding and feel like they have a substantial filling in them as they weigh a tonne!)
Celestion SL6
http://www.audioshopper.com/sl6a.htm