With no reoffer of the discount it's now a total no-brainer to take them to the Traffic Penalty Tribunal because the penalty does not increase, and there are no additional costs. If you lose, you pay the £70 they demand, if you win you pay nothing. They, however, have to pay the adjudication fee, and also prepare an evidence pack, so I suspect when they know you're going to adjudication, they'll fold and not contest.