You get £10 back on a £35+ Heathrow Express purchase. The deadline to use the deal is 19th May. There is no guarantee that this works with cumulative purchases but it is worth a try if you are buying Heathrow Express tickets anyway.
A handful of people are reporting a less generous offer of 10% back, but £10 appears to be the most common version.
Remember that you can also earn 10 Avios per £1 spent on Heathrow Express tickets if you use the heathrowexpress.com/avios landing page. This is more attractive than the standard rewards offer of 1 Heathrow Rewards point per £1 spent.
(Want to earn more miles and points from credit cards? Click here to visit our dedicated airline and hotel travel credit cards page or use the ‘Credit Cards Update’ link in the menu bar at the top of the page.)