SMEG built-in double oven SP 761 B grill problem?

The cooker works in all ways perfectly, except for the fact that the grill (in the lower oven), once switched on, goes up to full power and stays there, not controlled by the "knob". I've been told that there is not a normal thermostat for the grill element - and I can't see one mentioned on the wiring diagram.

I'd be very grateful if anyone out there know the answer to this problem.

Many thanks.