Hi sorry to hear about that problem.
What .you need to replace is the main knob control.
Please copy and paste this web site into your browser.
They can direct you to the part depending on your exact location:-)
I will help you with the replacement when you have it.
For that reason and since this will take some time, if you will rate my service as Good or better before you close this page, the system will allow me to schedule FREE follow ups with you so we can remain connected until we have this fixed for you. Below is the site you need:
The Main Control Knob is a Potentiometer (£13.25), are you sure this is the problem or is it the Control circuit board (£110.75)?
If it is the later then I will buy a new oven