There's no way to do it online because there is no FastTicket option in Swansea, so I went down to the railway station to buy it today. I opted for one leaving 18:56 going via Cardiff with Central Trains from there to B'ham, as I can't leave much earlier, so the Wales trek would be a waste with no daylight. I got it for £14!
Thanks a lot for all the info. This makes coming back to Wales at another time very appealing - I had a great stay here and am gonna se Oystermouth (i think) castle tomorrow.