I find it a pita as well but life is full of shoot like this. I downloaded the Ringo app. Really easy to use. Obviously doesnt help if you have no phone.

Next time take a couple of those really heavy duty rubble sacks and some duct tape and put a bag over the machines and tape them up. Only if not working of course.

I have to say, the two scooter wardens are fairly good in my experience. I had this issue a good while ago and one of them went and got a ticket from the other machine for me.