I've got a response back from the seller, Copy pasting it for further discussion.
That is nice of him to have offered a refund.hi, very strange, as i tested them before i packed them.
please let me know what refund you will be happy with.
I would have thought 10 to 15 GBP was fair considering, we don't have to ship anything back to him, so we save on shipping etc. Also, we may want to allow some benefit of doubt for him.Typical brit - parliamentary answer!
IMO, GBP 30 (50%) to be fair!
Just my two paisa though, What are your thoughts audio_engr, helium_flight, teky, others?