How can i adjust the thermostat of my Servis gas oven?

My Servis gas oven is burning food on the bottom yet it's uncooked in the middle.
I am going to buy an oven thermometer tomorrow to check the thermostat but suspect that it is incorrect.
I've taken the oven temperature control knob off but it does not seem obvious how I would adjust the thermostat if it is incorrect.

Does anyone know where I should look to adjust this or do I need to call an engineer? I hope it's the former...
Thank you
Esther