I've read a few of the replys on this thread, so I thought that I'd
throw my 2 bits in. 
>  At the moment I am worried about one of my Cephalotus. Most of
> leaves and pitchers have withered and died, while one tiny
> rosette has survived in the middle. Can any suggest why this
> may be happening?  >
Russell, this happens to mine occasionally however it is normally the
older crowns that die off while the ones that were produced more
recently keep growing.  I assume from the above that all the young ones
have died off and the oldest is still alive.  Is this correct? 
The only fungus I have found to be a problem is powdery mildew.  This
appears as a white powdery lesion (strangely enough) on the leaves and
pitchers.  The fungus damages the leaf tissue and if it is cleaned off
there will be a discoloration in the area of the lesion normally
reddish/yellow.  This fungus loves warm, humid places. 
>  I also need to repot my Ceph collection, and was wondering if
> anyone could give a mixture the find effective. Is it true that
> re-potting often sets back the plant markedly, and it often dies
> down after this treatment?  >
Peat/sand works fine.  I think the percentages of each vary according to
how I feel on potting day :) Normally I repot because the clump needs
splitting.  The plants do tend to die right back when repotted and they
may be slow to reemerge but I haven't lost too many.  I repotted some
about two months ago and a couple of them still only have a number of
very small leaves around the crown.  I'm confident they'll return to
their former glory. 
Cheers
Terry