If the gas rings are giving full pressure then I would suspect that the thermostat is the likely cause - I doubt it is a service item and would just need changing by a gas fitter. Try rotating the knob about twenty times to see if the thermostat shaft is just stuck, you may get lucky and it frees up. Check also that the plastic knob has not broken on the shaft and is just appearing to move the thermostat shaft, pull it off and use a pair of pliers to rotate the thermostat shaft - if you get high flame then its a cheap plastic knob that needs replacing.
It could (as mentioned above) be the jet - some can be cleaned with a rag soaked in solvent - some can be cleaned using a special wire for the job, dont try using anything other than the correct tool for cleaning - damaging the jet causes even more problems.