Trusts + form SA404

I am hoping that someone on here may be able to clear something up for me. I am acting for two PR’s in connection with their mother’s estate. The mother, during her lifetime took out a Loan Trust and was told by Skipton Building Society to register it. The PR’s believe that their mother did this in or around 2021/2022 as one recalls her mother handing her a sealed envelope to post to the Revenue enclosing a form. The PR does not know the contents of the envelope but has provided me with a copy of SA404N which are notes for completing form SA404. When I google SA404, something about carer’s allowance comes up but SA404N refers to the registration of a non-taxpaying trust. My clients are sure that the trust has been registered but do not have any documentation to prove this.

Has anyone come across form SA404 before? I was planning on writing to HMRC and referring to this form and asking if the trust has been registered but would appreciate any tips anyone may be able to provide and if anyone has found themselves in a similar situation. Thank you in advance, Sarah