oven ignites first time everytime,release know oven goes out?

my cooker hobs, grill etc all fully functionall how ever the gas oven ignites and the stays on( i can see healty flames on the burner )so long as i keep the knob pressed in but as soon as i release the knob oven goes out i have tried keeping the knob in for a period of time but no joy as suggestion i think its a baumatic model bt2790 still in almost new condition has been working for some 10yrs