(EDIT: This offer has been extended to September 30th)
British Airways and Heathrow Express have joined up to offer a FREE upgrade to First Class if you book a Heathrow Express ticket before August 31st. You can travel at any time until the end of November.
This offer is ONLY available if you book via the dedicated Heathrow Express / British Airways website. The good news is that you will also receive a handful of Avios points with your purchase.
The discount only shows if you click through from ba.com (Heathrow Express does this quite often these days with special offers). You need to visit this page on ba.com and then click the link to go to the Heathrow Express site.
The (minor) downside is that the upgrade is not genuinely free. You can currently save 12% on a Heathrow Express ticket by using the special MasterCard offer I outlined here.
This means that the upgrade is costing you the 12% differential in ticket price between the BA site and the MasterCard site, albeit this is offset by the 100 Avios earned.