parents immigration status

Hi everyone I am looking for advice on how to prove my mother had indefinite leave to remain when I was born?.I have already sent the passport office the passport she travelled here on which was her older brothers(my uncle) passport because she was a minor at the time.It has the ILR stamped on it but the passport office says that it only proves my uncles status and not my mothers.My mother is died before we new this would affect me.I no my mother would have no problem with her status because her mother(my grandmother) was british. I just don't no how to prove my mother was free from immigration.Any advice would be great.
thanks

Were you able to get any of the documentation together? If so, what documentation do you have? You may be another example of a surprised Brit.