Rangemaster 110 High burner malfunction?

I have a Rangemaster 110 Cream NG 5455 purchased in 1996. Excellent piece of kit and lovingly cared for. However, the high burner will now not go 'high' and the indicator light on the front panel occasionally illuminates combined with a clicking from the back. British Gas cover repairs and have told me that the problem lies with a connection immediately behind the knob on the front panel and that this part is no longer available. I'm not convinced that the engineers diagnosis is correct. Has anyone else experienced this problem please?