Thanks for the advice.
I have let BA know that they have overcharged me although I have to actually go into a travel shop with my ticket and rerouting fee receipt to let them know exactly what I mean- understandably, no-one wants to commit any responsibility over the phone.
My only worry is that BA in London are notoriously bad at understanding rules of OWE tickets etc- I guess it may be a long-drawn out affair to get my $225 back.

Chiym, I would have normally done what you advised and cancelledthe original ticket, but
1) I made the routing change AFTER departure, so cancelling the whole ticket would have incurred a large fee.
2) Also, the price of the ticket has gone up since I made the original booking (34,000 SEK to 47,000 SEK on Oct 1 of this year).

Jed