I called East Coast. They say the refund has to be from the station where the ticket was purchased however they have given me the following address:
East Coast Travel Centre,
Central Station,
Neville Street,
Newcastle
NE1 5DL
They say if I apply for a refund at any other station then they should forward the details to the station where the ticket was purchased. However I think it would be best to post everything directly myself. I didn't try to explain everything on the phone as the line was not good and I could hardly hear the operator. Will let you know what the outcome is.
What did you specifically ask East Coast? Did you mention the circumstances of the sale?
The additional ticket has been correctly sold, and used, and therefore cannot be refunded at the station.