Leisure Rangemaster 110 - problem with seized gas controller?

The gas controller knob to the main oven has become seized and will now not turn, basically reducing us to one oven. All other controllers to the smaller oven and hobs work just fine and move easily, it's just this one that's the problem, probably due to the fact that we've used it the most. I'm sure it's just a lubrication problem but access looks complicated and time consuming. anyone have experience of this and is it a long job (therefore expensive)?