Thanks everyone to answer some of the points made above, Twitter did get back to me after an hour or so and then phoned me and I cancelled. The email I had said that the booking could only be cancelled by phone which is why I hadn't tried to do it on the website - perhaps their email needs to be reworded. I know I would have had a couple of hours this morning but I had an urgent work meeting so would not have been able to phone and didn't want to miss the window as it was a non refundable ticket so I only had 24 hours. This will teach me to try and get ahead of things and book flights when Mr S is away on business!