Instructions:Visit the www.heathrowexpress.com website
Enter 'fcsep08' in the promo code field
You will be shown the discounted prices for first class (16.50 vs 26.00)
This deal is meant to be for people signing up for the Heathrow Express e-newsletter - but it works everytime. This code worked last month as 'fcaug08' - so you can prob experiment with it and get it working in the future!