In Pizza Express pizzas in the supermarkets at the moment there's a voucher inside. Use this voucher to get a main course for £2.50 when you buy another main course (cheapest meal is discounted). Not valid on Saturdays throughout the UK and on Fridays too within the M25. Voucher valid until 03/07/12. Not valid on takeaways. Few other restrictions like certain locations.
I bought my pizzas in Sainsburys and paid £5 for two small pizzas. The voucher is inside with the pizza but can be seen from the outside.