Tricity Marquis electric cooker heating control ?

I have a problem with our

Tricity Marquis 2502 electric cooking range.
Serial No. XM 1074 /3993. It is 12.9 kilo watts, 240 V

When we switch on the mains power supply, one burner (heating element/ring) automatically comes on. If we turn on the knob to use a second burner, then all four burners get activated!

The problem may be with the wiring in the unit behind the front panel with clock and controls. Can this wiring unit be replaced? If so, what is the name of the spare part that would be needed? Or might it be the simmerstats (the control device with knob that we use to adjust the level of heating) that need to be replaced? There is a part called
"DUAL CIRCUIT SIMMERSTAT - NOW ER1402" for Tricity 2502. could this be the thing to replace?

Thanks for your help