"La di da Limey" <drbryant@blueyonder.co.uk> wrote in message

Just guessing on this but since they've gotten in the habit of doing their
business in the dirt of the garden, you might try mixing some of that dirt
(for smell purposes) with some unscented kitty litter in their tray. Then
gradually decrease the dirt until you've gotten down just to kitty litter.
If you want the scented stuff, then you might try reversing the process by
adding a little bit of the scented stuff at a time until that's all there
is.

I suppose you could try it with scented litter and dirt to start just to see
their reaction, but if it doesn't work then you know what to have to do.