My cat keeps weeing behind the sofa, how can I stop her?

[JustFabulous]

We clean with the rug doctor whatsit. Have sprayed urine eliminator. Put down orange peel (just weed on top of it.)

She is old (18) and has several illnesses but still has huge quality of life and is still very entertaining and naughty.

I really want her to stop weeing in my lounge though.

Constant access to garden available.

Can you pull it out a little and fit a tray behind there, Just?

She can't aim and would still wee on the carpet.

How about those floor pads that someone mentioned - the on-purpose ones. (Can you tell I haven't got a blithering idea how to actually stop it?)