I've uploaded top and side view shots. You should be able to see the roots at the top dying out. Sorry - they're not the best qulity pictures. Thanks for the responses so far.
Ellen - I had considered that the humidity might be the problem. Our place is quite dry and the plants we have generally suffer if we don't mist the leaves daily. I generally do the same with the orchid. I'll have a look for those posts on bamboo skewers
Cynthia - that's an interesting one. We do have slightly hard water in our area. I haven't tried a fertilizer yet, perhaps that something I should do. What baffles me is why the orchid would be fine while it was flowering (we even had new roots growing over the top) but then start dropping off
