Hi IWBY
The first thing to establish is whether or not you have any case.
Look at the document you signed when you fixed. Did it make it clear that you were giving up your tracker? If it did not, then you should have been offered a tracker when the fixed rate ended.
Look at the options you letter you got when your fixed rate ended. Was there definitely no mention of a tracker on it?
If you were entitled to a tracker, then, I would argue that the six years started when they stopped overcharging you , which was presumably the date you switched to BoI. UB will argue that the six years started when your fixed rate expired.
In any event, if you have a claim, get it into UB immediately and then to the Ombudsman as soon as possible.
Unfortunately, the Ombudsman has in the past determined that if someone switches to another lender, that ends the former bank's liability to them. If the amount is big enough, you might consider the High Court instead of the Ombudsman.
Here is an example worth reading
UB Tracker Return, but capital repaid because I was on SVR, FSO awards compensation
Brendan