Heathrow Express has provided a promo code to get 25% off their fares until 23 December 2016.
Go to their web site, enter the code HEX25 and voila, you get a discount.
Terms and Conditions:
*Valid on full price, Express class, single or return tickets purchased via Heathrowexpress.com or the Heathrow Express Mobile App before Friday 23 December 2016. Valid for travel dates until Friday 23 December 2016 only.