How can I complain Stansted Express? (London)

Dear All,

I had a flight on the 25th of December (Saturday) from Stansted Airport. I knew that it was Christmas day and London transportation wouldn't operate. However I checked Stansted Express web site last month, couldn't see any announcement and tried to buy a train ticket for the 25th of December. System let me buy the ticket therefore I thought that it will work on Saturday so didn't worry about it after printing a copy of my ticket. When the day came, I went to the train station (Liverpool st.) and it was closed!
Then I had to take a cab which costs Protected content ! I took a receipt from the driver. Now I'd like to complain about it and ask my money back.
What's the right way to do that?
Thanks in advance.
ilker