half of The girll wont turn off on my oven?

I have a tricity double oven, quite old I think.

It has a facility to have all 4 elements on the grill or just half the grill ie two elements.

my flatmate was using the full grill and put the switch all the way to the top setting (5) and then it clicked past the 5. Now when we try to turn it off, two of the elements stay on.

To turn them off, we have to now turn off the main oven switch on the wall which is a bit of a pain.

any ideas?
thanks