Just a final note on DUB, I used it a few times and the market certainly wasn't purely local. Always a good proportion of Irish accents (I know that doesn't prove they live in Ireland), but people heading for the train is a good proxy measure for those heading outside of south-east Essex and there were always a few.
Also for the record I have no vested interest in the route returning - LCY and STN are just as convenient for me.