Gas oven lights ok but turns off

We have a Parkinson Cowan Ovation 60GLXa gas cooker with electric ignition on the hob and auto electric ignition in the main oven. The cooker is about 18 – 20 years old I guess.

Used the oven last night without any problem; today the oven lit as normal but upon turning up the thermostat control, as soon as it reached the number 5 the oven light and burner went out. Turning it back down again, it re-lit as the control came down to No 5.
Turning it down further, the same thing happened when the control reached No 3. Tried again & again and this happened every time.

Leaving the control between No 3 & No 5, the burner and light stayed on and we were able to cook ok at those temperatures; the thermostat worked in as much as when the optimum temperature was reached (for setting No 5) then the burner died down as it should.

If I know what the problem is then I can ascertain the cost of repairs and if worth spending on.