The company behind Tube Refund, one of the apps, says that there were approximately 1,996 delays across the Tube network last year (38 delays a week). Hopefully you're not going to get that amount personally, but the which only costs 59p, could pay for itself pretty quickly.
The other app iRefund is $0.99 has fewer ratings than Tube Refund.
If you have an iPhone give one a try, when you're next delayed. They'll make a useful edition to the other London Underground iPhone apps reviewed last year.