how do you fit a thermocouple to a myson boiler?

I have a myson boiler marathon 700c I wish to fit a thermocouple There is a clamp holding a gray tube coming from the gas valve and a thin copper tube which I think is the thermocouple' I removed the clamp which is held on by two nuts the rubber bung also comes out this leaves a hole about 3/4" in dia with the thin copper tube going into the hole I thought it would just pull out but it seems to be fixed. how do you remove it from the hole?