Replacement knob for main oven for Henley Professional 600 cooker 10688G ?

The main oven temperature control knob, on my elderly parent's Cannon Henley Professional 600 cooker - Model No:10688G - has broken, rendering the large electric oven unusable . The cooker is 10 years old and replacement knobs are no longer made. The cooker has been well looked after and is in excellent condition - any suggestions as to how the problem with the main oven can be overcome so that they can use it again?