The control knob is actually the thermostat, on my boiler its good up to 110*C. if you BIAB or mash in the same vessel then the best thing is to replace it as you need to maintain your temperatures. You can get genuine Burco parts on line. I'm working on this just now, as I picked up a 20L free boiler the other day as the thermostat is knackred. The wiring is very simple to follow and as has been said heat resistant wires and crimps are easily available. If you feed google with the model details etc... you might get a wiring diagram, there is even a thread on here discussing this very issue.
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Don't be frightened of repairing this yourself, it won't be plugged in when you change the wiring, all you have to do is trace the wiring and make a sketch of it before you start so you know where everything goes, its worth the effort, it really is.