You may be able to contest her will if it fails to honour a promise made to you, and you reasonably relied on that promise to your detriment. For example, if you helped your mother buy her house by contributing part of the purchase price or were paying the mortgage (particularly if your mother asked or encouraged you to do so) you may have a legal claim on the house. It does not matter whether the promise was made orally or in writing — although it is obviously easier to prove if it was in writing.
You may also be able to enforce a promise your mum made that you would have a particular asset when she died if:
She made it ‘in contemplation of’ her death — for example, when she believed she was close to death.
She passed control of the asset (for example, she handed over title documents to her home, or the key to a steel box she kept them in) to you.
The asset remained hers in the meantime, so she could revoke the promise at any time.