I am sorry Henry has this problem.
It is great you are doing weekly water changes, make sure you are dechlorinating the new water with a dechlorinating tonic or are leaving the fresh water to stand for 24 hours before putting it in (this removes the chlorine)
There are many things that could cause this lesion from a parasite, to a tissue mass and possible chemical burn from reduced water quality.
I would get your water tested at your local aquatic centre to ensure the ammonia and nitrate levels and pH are appropriate.
It might be worth switching from an antibacterial tonic to a more general antibacterial/antifungal and antiprotozoal tonic.
If the water quality is abnormal you may need to start more aggressive water changes.
Adding salt is good for debilitated fish and can help them recover.
If things aren't improving or no water based cause is identified, you may need to have a look locally for a vet that deals with fish. If you look on the Royal College of Veterinary Surgeons website you will be able to search for this http://findavet.rcvs.org.uk/home/
It is normal for goldfish fins and scales to change colour but usually it is from black to gold.
It is possible this may be a conditon that isn't able to be cured but it is definitely worth having a go with the above procedures.
Good luck and I hope Henry is OK