OK so I repotted today. When I removed the old mix it was very wet. The new mix was of sphagnum (dead), perlite and orchid bark in more or less equal quantities, so more or less as close as I could get to your mix Whimgrinder. The only living sphagnum I have I keep for dressing the pots. Pics here:
Notice on the sickly hamata the original sphagnum plug it arrived in. I left this intact as I did not want to damage the roots, was this fool-hardy? So I couldn't really see if the roots had died back. On the 'healthy' plant the roots hadn't grown beyond the original plug either, so I didn't think I was justified in panicking about damaging the roots Corky! Is it the case that the wet mix prevents the roots growing and developing?
As you can see the new mix is very open. The reason for this is I 'd prefer to mist thoroughly every evening to keep the moss lush. How do other members keep their sphagnum happy without the plants themselves getting too wet? Is their such a thing as being too open, and how long can I realistically leave the plant before re-potting again bearing in mind the frequency of misting? I think you are right Murphyslaw, I'm gonna gradually re-pot all the plants into this new mix. Thanks for all your help!