Hi, I'm Dr. Jamie. You should be proud! To be diabetic and 18 years old your cat has put forth real effort to potty in the garden until now. I would agree that the reason L L has stopped using the garden is either because he smells the aggressive cat nearby the garden or he most certainly has arthritis (as most 18 year old cats do) and it is just too uncomfortable for him to make the trip.
I would suggest offering him a litter box near the kitchen while you sort out the other possible issues. For the possible arthritis a glucosamine supplement can be very helpful in making cats with arthritis more comfortable. I have personally seen good results with this one called arthrimaxx: http://www.amazon.com/ArthriMaxx-for-Cats-6-ounce/dp/B00076NNRO
You might also talk to your vet about trying a pain killer to see if it makes a difference. There is one called Metacam sometimes used in cats. Secondly, if there is anyway you can keep the other cat from coming around, that would be helpful. You might also try spraying a product called Feliway in the area. It is a cat Phermone that causes cats to feel calmer. you can see it here: http://www.amazon.com/Feliway-Plug-In-Diffuser-bottle-Milliliters/dp/B000WHUOEI/ref=sr_1_3?s=pet-supplies&ie=UTF8&qid=1402952962&sr=1-3&keywords=feliway
Thank you for your help. A few years ago we tried using a litter tray when the weather was particularly bad but Little Cat didn't seem to know how to use it. Is there anything we can do to encourage him to use one now?
When the aggressive female cat comes to the garden LC makes a very
loud throaty sound that we have never before heard him make. She is very forward and sits in the sun on the bench L loves. Would spraying the
fence help repel her? Are neighbors with cats are complaining about her too.
I'd just like to ask one last question about this throaty, loud growling that we have never heard before in these past 17 and three quarter years. We didn't even know Little Cat could make a sound like this.