Dog Peeing in Its Own Bed?

I have two British Bulldogs and my eldest, who is now 11, started peeing on her bed when we got the younger one. Initially I thought it was a dominance thing, however, it's been 5 years now and there's no sign of improvement. Instead now she doesn't care if she lays in it.

We let them out regularly and for longer last thing at night and even take the water away after 6:30/7. I have even tried not washing the bed just drying it out, but it made no difference. She does not do it during the day though it's only at night. Any advice welcome :)

Thank you.

By Carina