If they don't ask for it back, or they havn't invoiced you for it, then you could keep it, I'd wait for a few weeks at least.
AFAIK you can claim the extra stuff was a "gift" under UK law or something like that.
Or if you are a good, honest person you could send it back, in which case you'd have to pay the carriage, then they SHOULD refund it, this way is more hassle
I'd keep it, serves scan right imo