Just posting this in case anyone else having the same problem.
I'm going to this but my mate isn't . I buy his tickets as ' friends and family ' . So I tried to claim just one seat as an STH but there is no option to remove the other seat from the basket.
Had to make 3 phone calls to EFC ,totalling 80 minutes mostly in a queue. Now have spent more on calls than the £15 ticket I reckon.
Eventually they decided to sell me the one seat over the phone. Apparently this particular glitch will be sorted soon........