If your mother sought, and got, probate then the 'grant of representation' will be a public document, which anyone can get hold of for a tenner:
https://www.gov.uk/search-will-probate
However if you've found your father's
original will, rather than a copy of it, it seems unlikely that your mother ever sought the grant (as the original will would then have been retained by the Probate Registry).
While the people at your local Probate Registry office aren't permitted to give detailed legal advice, they can provide general guidance on situations like yours and they're normally very happy to do so. So they should be the first people to ask.