Back in the olden’ days (ie pre 2013), you could always rely on the internet to find you a nice promotional code to reduce the cost of your Heathrow Express ticket ….
From the start of 2013, though, all of the discount codes except one (for Air New Zealand) stopped working. And then, at the end of January, the Air New Zealand code died as well.
Some group saver tickets are still available – see this post for details – but nothing for single tickets.
However, Flyertalker monkeymagic has unearthed a working Heathrow Express promo code – targetted, wierdly, at the Spanish Chamber of Commerce in Great Britain!
The link is here (EDIT: link removed as now dead)
It gives 10% off economy and First Class tickets, taking a single economy ticket down from a ludicrous £20 to a only-slightly-less-ludicrous £18.