If you believe your Mum isn't sound in her mind, then this would need to be confirmed by a Psychiatrist (referred from her GP).......Otherwise, the Law considers her to be sane & she can change her Will as she chooses (even if she's being pressurised by a con-man or SS).
The Mental Capacity Act is designed to support mentally infirm people, but it's been my experience that Social Services are working it to their advantage.
If you can, get a solicitor quick......CAB are good, but ineffectual because you're still on your own.....