We don't use flubendazole too much in the US, but it should work. It is toxic to invertebrates (unlike praziquantel) so you can only use it in a fish-only tank, or in a hospital tank. Use the dosing amount as per the label, but use the 7 day spacing between treatments, with a partial water change prior to the second treatment.
Reduced the dosage of copper and started the fluke medicine and have turned the lights back on Saturday morning. To my eyes at least they are starting to look a lot better and hopefully we are at a turning point with this.
Thank you for all the advice so far, and if I am wrong please let me know.