It shouldn't be any surprise
This is one of the worst websites out there. I recently tried to purchase a pair of tickets from Cardiff to Manchester. Every time I got to the payment stage, it told me that the card had failed authorisation. I checked with my card issuer and they hadn't even received a request, let alone rejected it. I spoke to the 0870 number people and got queued, then told that I would have to empty out my shopping basket and start the whole process again. Did this more than 6 times, tried three different credit cards all with the same results. The operators on the line were, shall we say, inexperienced and reading from scripts, anything outside their comfort zone was not something they wanted to be bothered with. After three hours and calls to their 0870 number and two of my card issuers (just in case!) I gave up. I tried once more the following day with the same result, then phoned them, spoke to a very helpful lady who took my order over the 'phone and used exactly the same payment card as I tried first which worked first time.
Definitely not a candidate for Best on the Web!