Started using my kitchen as a toilet at night. They are let in garden at bed time to do their business then locked in kitchen over night. They are three years old, and didn't do this when younger, don't know what's changed. Any advice would be appreciated. This has been going on for at least six months.

Your Jack Russells are not happy about being locked in the kitchen and are showing it the best way they can. Smart little critters those Jack Russells. You will have to do the process of elimination here and see if they want more human interaction such as play time before bed. Have they always been shut in the kitchen ? They are completely adult now at three so have all their faculties in full pay as only jack russells can.

Tick

Hollow, I have a number of questions that might help you solve the problem. Were there ANY changes in the household around the time these incidents first occurred? If you can't think of anything immediately, give this a lot of thought. Younger dogs don't usually just change a regular habit unless something has occurred that disrupts their routine.

Other questions: Are you sure it is both dogs and not just one? Have you had the vet give them a health check since this first occurred to rule out any physical problems? When "they are let in garden at bedtime", what time is bedtime? Do you watch them do ALL their business or do you just open the door to let them out and then let them back in when they show up? What time do you feed them? What time do you pick up their water bowls in the evening? OR, do you leave them in the kitchen overnight with water and food bowls filled? How many hours are they left in the kitchen without attention/walk... basically, how much time passes between when they are let out in the evening and when are they let out in the morning to do their business?

My dog Owen is still a puppy so this may not be helpful. In case it is I will share it.
Owen went through a time of pooping in the house every night. He was let out at 9pm and then again at midnight and around 7am. We got tired of it and put him in his kennel at night. We kept taking him out. He has not made a mess since. We now can keep him out of the kennnel. I only had to use the kennel for about 2 weeks.
Someone had once told me a dog will not dirty their kennel unless they are sick.

Mogrann