Can I replace Broken Dimplex CXLN control knob?

I have a Dimplx CXLN storage heater (about 15 years old) fitted in my flat and the input control knob has broken off due to the plastic drying out. Is it possible for the knob to be replaced? The plastic has snapped off with the rest left inside the metal setting. My landlord has only looked at it briefly and said to that the whole unit will have to replaced just because the knob is broken! Would it be possible to replace the knob and not the whole heating system?