I too had tinnitus for a long time and it all went away after chelation treatment against chronic mercury poisoning from amalgam fillings after being lucky that it was diagnosed (I had to actively search for a rare researcher/university clinic doctor specialist, nearly no chance with normal doctors). See my post history, I mention it a few times. I could write a few more pages about it all but in this format I'll leave it at that, people with the problem can decide for themselves if this is relevant for them. I was reluctant to post because I know a lot - most? - people don't take low-dose heavy metal poisonings seriously at all. I had people wanting to buy an espresso machine off of me despite me having revealed that I planned to scrap it because the water coming out of it had twice the official limit for lead in it (and the medical limit is zero so any lead would have been bad). Because "how bad can it be, such a tiny amount sure does not matter".
OP is from Scotland and I though it might be relevant because it seems they use that stuff according to this very recent document: https://www.gov.scot/publications/amalgam-dental-fillings-ac...
PS/EDIT:
By the way, my tinnitus went away within year one of chelation treatment. However, after almost a decade (of continued occasional chelation because it still helped) I had a lot of activity all of sudden for a few weeks around both ears. For the last decade I had shifting activity patterns in various areas, so that wasn't a surprise, I had expected something like that since that was a major impacted area. For weeks I could not listen to music and definitely not use a headset. Something was going on there and it wanted quiet.. Anyway, long story short - I can now hear much better than a decade ago! I first noticed because I've had the exact same MP3 player with the exact same songs for well over a decade, and the same in-ear headset for just as long. Suddenly I had to turn the (numerically adjusted) volume much farther down, into regions where previously I could not hear a thing apart from "music plays". Now I have to play everything on about 10-20% less volume than before, and it all happened exactly after those weeks of activity (where I even had to go to a doctor to get the ears cleaned, there was way more ear wax than usual during that time).