gas oven (NG) overheating?

we have a Leisure Kensington S1 cooker, the oven works fine but will only cook on full blast and not reduce the temperature at all.

Any setting between 1 and 8 just keeps it going full blast and does not reduce to the setting heat.
The S setting when set manually works fine.

Should I be looking to replace the temperature probe in the top of the oven or looking elsewhere..??