I have exactly the same Titanium X model and vintage of Focus as Ian with exactly the same problem.
I added the driver assistance pack to the car. It's supposed to pick up speed limit and other signs with the on-board camera and display them in the instrument cluster. I thought it would be a useful addition to the sat nav but what happens is that the sat nav doesn't display any limits and the cluster display is of limited use because the sign fades away quite (or in the case of the national speed limit sign) very quickly. So you can't have a quick glance to remind yourself as with a sat nav and the whole system doesn't begin to work until the camera picks up the first imit. Worse still, the little gizmo that tells you when you exceed the speed limit by a number you set doesn't work at all in national speed limit because the sign doesn't stay in the cluster for more than about ten seconds.
I should have just stuck with the sat nav, I think.