How do I stop my cat(s) weeing in their bed?

Washing the beds won't get rid of the original urine scent, my advice would be to just throw them away and offer new bedding if you can. One of them may have been upset with something and told you that by urinating in the bed. Of course after that, it's monkey see - monkey do with the marking scent still in the air. If they are usually good about their litter boxes and nothing has changed, try replacing the bedding first and see if it happens again. You could even use just some old blankets or towels before buying new beds. Good luck!