tricity bendix halogen hob?

My tricity bendix cooker is 20yrs old and is a Sovreign IV. The large halogen ring at the front-the one that has been most used, is temperamental and seems to come on only after one or other of the rings has been on for a while. Once on its difficult to get it back off as the knob keeps twisting round. When it finally is off, its not usually in the normal off position! Is any repair going to be cost effective as its in near perfect condition apart from this. Did wonder if there was a huge amount of gunge stuck under the switch-maybe needs to be taken apart and cleaned?