My Delonghi Quattro Plus Calor gas heater will not stay alight unless I keep the control knob pushed in

I cannot see a valve listed as a spare part only a tap and an analyser I cannot see a valve listed as a spare part only a tap and an analyser

You will find the problem is the thermocouple its the small bullet looking piece that is by the pilot light..What this does is turn the gas of if the flame is blown out and when the thermocouple cooles down it shuts the gas off but if you have a faulty one it thinks its cold all the time and will not allow gas through the valve after you release the knob.If your handy there easy to replace and fairly cheap ebay?? You will find the problem is the thermocouple its the small bullet looking piece that is by the pilot light..What this does is turn the gas of if the flame is blown out and when the thermocouple cooles down it shuts the gas off but if you have a faulty one it thinks its cold all the time and will not allow gas through the valve after you release the knob.If your handy there easy to replace and fairly cheap ebay??