Originally Posted by
stifle
I don't know if you can. The machines don't do them and I haven't seen a HEX office at T5. Which should entitle you under the National Rail Conditions of Carriage to buy the discounted ticket on board, but I've no idea if they'll allow it.
(The discount is actually 34%, but is less for single express-class tickets on weekdays as there's a minimum fare of £13.)
It's on Sunday I'll be coming through - so would be £12 I estimate as it's £18 from a ticket office, rather than £16.50 online. No idea where the damn thing is though.
Given that, the 25% off above is almost the same deal - £12.38 and much less faff and paperwork.