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To finish the story, we had contacted various lawyers who
1. Said that as mum can't speak to confirm her identity they won't do it
2. As they can't see documents in hard copy they won't do it
Finally I found someone who agreed to see the docs by e-mail, then in zoom call. They needed I was also lucky that we had Freedom pass with 2 names and a photo identical to a photo in Chilean passport with all 4 names. Plus council tax with 2 names, pension statement with 4 names but with amounts matching the money coming in to the 2 name BOS account. So 2 weeks and 7hrs on the phone later and another £ 350 lighter we have finally managed to get it all registerred and made first transfer!
From Monday we are on attack, another bank, GP, WDP, council, housing, private pension, social services, utilities, and there will be probably few more, which I am not aware of right now. I wonder how long it will all take.