I"ve given up for today.
It said I had a credit of £51.50 but wouldn"t let me use it against my ST purchase. I tried again and then it said I didn"t have any credit even though I haven"t used any of it.
My mate has just experienced exactly the same thing.
I"ve emailed ticket sales and got a reply saying the office is unmanned but someone will get back to me as soon as possible.