I have an intermittant fault with the lock on my Double glazed backdoor. the inside key at times jams and it will not turn without a lot of fiddling. I want to change it for a new barrel and thought it would be just a case of undoing the three screws, pulling off the handles and tapping the barrel through. However having only given it a gentle tap with a rubber hammer it will not budge, is there a knack to getting it out ?