Fault F04, Bosch exxcel 1600?

We have an error mssgae F04, and water in the drum as the machine has not completed the wash cycle. Having read people's advise last night we attempted to remove the bung on the front left of the machine, but could not! The plastic bung appear fixed tight! Can anyone advise best way to tackle this and confrim that this is th pump bing everyone referrs to. We were confused as to whether is turns and threads all the way off or a small turn an pull, either way its stuck tight! Many thanks, all suggestions welcome.