DIAGNOSIS The control knob is attached to a potentiometer (a variable resistor) behind the panel. The wobble means the plastic stem of the knob itself has worn out or cracked where it connects to the metal shaft, or the retaining clip inside has failed.
FIX You can often fix this without opening up the electronics. 1) Unplug the oven. 2) Gently but firmly pull the knob straight off the front panel. It should come off with steady pressure. 3) Inspect the inside of the knob. You'll see a D-shaped or splined hole. Look for cracks in this plastic. 4) If it's just loose, you can add a shim. Cut a very small, thin strip of electrical tape and place it over the flat part of the metal shaft. 5) Push the knob back on. The tape will take up the slack and reduce the wobble.
IF THAT DIDN'T WORK If the knob is visibly cracked or the wobble is in the shaft itself (meaning the internal potentiometer is loose), you need a replacement knob or a service call. Contact Breville customer support with your model number; they can often send you a new knob kit directly.
HEADS UP Avoid turning the knob with excessive force or by gripping just the very edge. Use a gentle, palm-on-the-knob motion to select functions. Forcing it is what leads to this wear and tear.