No advance sign for a left turn, and the signs would only be visible as you start to turn in. So what do you do if you suddenly see them ? Jam on your brakes and cause an accident ? I don't think so, so submit representations based on lack of advance warning for a left turn into the street. They'll reject you, of course, because they always do, and only really want your money. So you'd probably have to take them to London Tribunals, where lack of warning has won in the past, but it is not guaranteed. So, its a choice of paying the discount, or taking the double-or-quits gamble at London Tribunals. The odds here are not too bad, but one never knows what an adjudicator had for breakfast; some seem to eat lemons !
