Can you post the model number please.
Don't know model no....where do I find it..can't find booklet
The model number is ***** the appliance but if you don't have to hand don't work about it at the moment.
As to the fault if the centre burner lights but goes out once you the go of the knob then what this will be is the thermocouple as this is the part that tells the thermostat the gas is lit so if it fails you get this fault were it lights but goes out once you let go of the knob.
To replace the thermocouple depends on the model but usually you need to remove the burners and pan stands and control knobs.
Then around each burner are a few screws. Remove them and then you can remove the hob to and there you will see the thermostats and the burners.
The thermocouple is the sensor probe which goes from the back of the thermostat to the burner. You need to replace that and then fingers crossed this will cure the fault and get it working again.