I had this on my Belling 110 on two rings and both were faulty simmerstats. The exact replacements weren't available, but I managed to find some compatible ones (less than £10 as I recall) with the same mountings and connections.
To fix, remove the front panel (screws at each end, make sure all power is off!), pull panel forwards (towards you) and rotate. Remove knob and undo the two external screws. Note all wiring connections and remove simmerstat. Look up the number on it, find a compatible (or genuine replacement, depending what you find) and refit. Same applies for the oven controls, although they're more tricky as there is a temperature sensor to be routed also.
EdG, November 2015
