Thanks to readers Nick and Jamie for spotting the latest ENO discounts.
Via Moneysavingexpert.com (scroll down to find the deal), tickets for The Elixir of Love are £10-£15 (Upper Circle) or £20 (rear Dress Circle) - and because they're sold via the ENO website, you get to pick your seat.
Meanwhile, it seems the reason you can't find any discounts for The Passenger is that ENO have been reduced to giving the tickets away. With good reason. Having managed to sit through less than an hour of it on opening night before boredom obliged my exit, I would say 'total dud' is polite.