Portable gas heater (Pont-dôl-goch SY17)

Alvima portable gas fire, requiring attention. As the picture shows, it is in good condition and working, but unevenly. The centre burner in particular never really gets going, therefore there may be some escape of unburnt gas, which could be hazardous. I have had it apart but my attempts to clear the pipework haven’t lasted. Takes a 15kg butane cylinder (not included).