Bank transfer gone wrong

Even if ths is not the case, it is his error that has caused this and under UK law he is liable to either supply the goods or return the money,

The OP acted in good faith based on the information "James" gave him. In court this would be no contest.
County court judgements are a crap shoot - This is nowhere as cut and dried as you make out, unless you have access to precedent?

I would be far more inclined to write to the bank, and request the contact details of the account where the money was paid.

There is nothing in the Data Protection Act that states that the bank cannot pass on these details.

You can always apply to the courts to order the bank to give you these details. Inform that bank that this will be your next action.

Alternatively, you can ask the bank to forward correspondence to the account holder.