Hi Claire
I am so sorry to hear the problems you are having . My cat Cammy was the same, and I sadly have to say that she never went back to pooping in her tray, only on the floor. nine times out of ten was in the bathroom or in one particular patch in the hallway, but occasionally she would do it in front on the bedroom door, pretty grim for late night loo visits!
Liek Purrrrfect said check her over at the vets to make sure is nothing underlying! You didnt say how old she was? As my problems started when Cammy was about 8, and even though she lasted another 10 years the vet did say was the start of old age!
You could try a feliway diffuser to see if she needs calming down,(an oderless kitty pheromenes diffuser), and the spots where she has pooped, clean with warm water & biological washing powder or hand wash stuff, then when dry wipe some surgical spirit over it as this really gets rid of any lingering smells! I hope this helps or at least reverts her back to pooping in the proper places
