How can I fix my old Thorn EMI Richmond-2 gas fire?

The Thorn EMI Richmond-2 (model 833-2089425) has a pilot and four push-button settings (off, low, med, high). Normally, after pushing in a button, the center burners light with a low flame and after 10-15 seconds the burners selected come on full force. As of last Friday the centre burners still light on low flame but do not come on full force. What is the problem? And can I fix it? Should I check for low gas inlet pressure? or suspect a faulty pressure regulator? or the thermostat? or check for blockage in the control valves or manifold? I think spare parts are no longer available.