How can I fix the temperature of my parkinson SIG454WN double oven/grill?

On my Parkinson Cowan double oven/grill I cannot alter the temperature on either the oven or the oven/grill? It is a Parkinson Cowan SIG454WN. The ovens work on high heat only and there is no variance when turning the control knobs. We have had all the knobs etc., to pieces and cleaned, adjusted etc., but still it will only work on the high heat. Any ideas and where can I get the spare parts from? Thanks very much for any help.