My OH and I booked a couple or so months ago to go and see Dreamcoats and Petticoats in London today.
We went on line and, through Saga found a company called encore.
We ordered two tickets at around £22.50p each (half price) plus we paid an extra fiver to make them 'flexible'.
Just as well we did because I have Labrynthitis and we had to ring up and cancel.
The £45 credit we got back from the tickets means we can rebook any time, and at any theatre covered by encore over the next 12 months.
Obviously it's cost us £5 but we could have lost the lot.
Hope that helps.
('encore' is phone number 02074921500 and based in London).