Didn't include failed module on application form

I failed a module in the final year of my degree and for some reason I stupidly omitted the result on my application for a technology scheme to a bank. Not sure why I did this, suppose I thought it looked bad. Is there any way they can find out - my official certificate only has my degree and grade whereas I only have a very unofficial looking printout of all my modules, which could easily have been faked. Not sure if I should let them know now after I've had my first interview and they invited me back to the next stage.