We had a problem with our ENO oven with it not going up to temperature. We knew ours had no thermostat but the flames did not seem as big as they used to be. After speaking to Gas engineers who seemed to know nothing and simply suggested a new oven I decided to try something myself.
I disconnected it from the gas supply. Had my wife hold the oven control open as if we were trying to light it. Then I held a bicycle pump to the gas inlet to the cooker and gave a few pumps. I then reassembled it and the problem was solved; large flames again and the control worked again to adjust them. I did get the connection tested again later.
Alternatively, if you really think there is a thermostat try contacting Penguin Engineering. They are an Eno agent and have helped me in the past with a part that I was uncertain of.