I decided to remove the dying leaf and the rest of the flower spike. I got a new sterile scalpel blade and cut away the spike. I pared away the dying leaf cleanly and as I picked the plant up the big leaf just snapped off!
I had a look at the base of the big leaf and there was definite signs of rot there too. So I decided to take the plant (now leafless!) out of the pot and check the roots again. I went through each one and they are all firm. I then decided to pare away the crown until there was no sign of rot. That done I sprinkled the whole thing with cinnamon and repotted in a freshly sterilized pot with fresh bark mix.
Here below you'll see what I ended up with.

Sorry this is out of focus but you can see the rot quite clearly at the base of the big leaf.
Marion