I have an annual z123 on my Oyster card and want to get a single on the Stansted Express (from Liverpool Street to Stansted). I expect I should be able to get a single from boundary zone 3 to Stansted with gold card discount, but will I get a maximum cash fare if I touch in at LST and fail to touch out, there being no opportunity to do so (unless I develop extreme sprinting skills in the meantime for use at Tottenham Hale)? If so, is there any way to get around this?