Hello and welcome My name is XXXXX XXXXX I am an experienced small animal veterinary surgeon, I will be very pleased to work with you today and will try my best to answer your question to your satisfaction.
There are some things you can try before you go to the extent of a behaviorist ...
In your sort of situation products such as Feliway can really help, here is a link about Feliway LINK
If you have not tried it so far then give it a go ...
Another thing that can help is to change your cat litter to another brand which is very different in colour and texture from the brand you are using now. Also consider using earth from the garden or peat from the garden center instead of cat litter, it is not quite so absorbent as cat litter in terms of smells etc. but it is much more natural for the cats and might bring them back into their litter trays. If this works then you can gradually switch back to commercial cat litter.
Just in case you are using a regular carpet shampoo instead of an animal one ... When you clean where they have been dirty use one of the odour eliminator type solutions you can get from the vet or pet shop. These work as they have a biological action like a washing machine soap powder and digest organic matter. In fact a dilute solution of such a soap powder is a good thing to use as well. If you just use normal soap and water or other shampoos it will smell fine to you but to the cats it will still smell afterwards, and if the place smells like a public toilet to them then they may use it as such! Make sure you leave the cleaning agent to soak for a while.
Well there are a few ideas, please get back to me to talk things over further ..
Thank you for your suggestions. We will try them . I can't put a happy face because the situation is so depressing.
If nothing helps my husband is saying the older cat will have to go which is too awful to contemplate.
We are implementing your suggestions and have found a local supplier of Feliway which we will purchase this week.
We purchased a CCCT camera with a monitor and identified the main culprit! Pelucia was the first cat in residence and has never forgiven us for bringing in Una 10 years ago. The monitor beeps when the cast comes into the affected area and she doesn't seem to like it as we didn't have a show last night.
One step forward at a time.