Writing Will with Dementia

Hi,
I was wondering if anyone could advise me please?
My Mum sadly passed away earlier this year, she wrote a Will with a Will writing company that has now unfortunately gone bust, we have the Will but no records referring to the writing of it as they can not be located.
My brother, though they have not really seen my Mum over the years has put a caveat on the execution of the Will stating that Mum didn't have mental capacity. I am unsure when she was diagnosed and we have applied for her medical records but I know her consultant has now retired and her GP is no longer practising in this country.
Could anyone please let me know how to go about getting a medical opinion regarding my Mums capacity at the time?
Thank you