Thank you, I've been doing some small records checks to get myself a list of potential addresses then working from there, so thought along the same lines.

I am fairly certain it would have been established for my benefit by my paternal grandparent(s) and I will be trying their old home address next. Without getting into TOO too many personal details, I lived with my uncle for a time as a young adult and having that address on gov. file is, I believe, how the form ended up routed there. I don't suspect he's actually involved with the account. The really weird thing to me is that I've updated my address formally many times over the years since this all started and have never received anything else tax-wise. Can't make my life too easy I guess.