National Express is doing a “free coach journey”, but you still have to pay a 50p booking fee and it doesn't cover all of the routes, and its limited to a few dates, but it still might be useful, and you only have 50p to lose if you change your mind about travelling. You have to be an o2 customer, so you can redeem one of its moments through the below link.

O2 & NATIONAL EXPRESS 50p FARES