Columnaris often imitates finrot, it could be that. The angel that isn't eating probably has an internal protazoan, which isn't too difficult to cure with meds available in the U.S.
The problem is med availability in the U.K. I use metronidazole for internal protizoans, which I am almost certain isn't available over in the U.K. I think dimtridazole is available to you, though you may have to get it through a vetrinarian. I could post my usual longish cure for protizoans if you like, some of it may apply.
If you could post a pic of the finrot situation, it wouls help. Do a search for columnaris on this site, I'm sure you will find a med that is available in your area.
*Edited for spelling, just woke up, need coffee.*