Boiler won't turn off. Can anyone help?

[hs1972]

We're having problems with our Baxi system boiler. When both the heating and hot water turn off on the controller the boiler stays on, and keeps firing. However the issue is that it does this intermittently, it will go for a few days and be OK and then the problem will start again. This problem doesn’t occur when water only is on. We’ve changed the controller but this hasn’t helped. Can anyone help? Our system is around four years old.

[TOHeating]

What is the model # ???
You have had a heating contractor in to look at the problem, or how did the controller get replaced ?