It may depend on what the papers you signed said. Presumably you do not have a copy. Who is living in the house? Who pays to maintain it, & for council tax etc.?
To do anything I think you need to see a solicitor. On the face of it, you are entitled to your shares but I don't know what the complications might be as a result of the long time lapse.
If you see a solicitor, take with you a copy of the Land Registry entry (download from the website), your mother's death certificate and any other documents about this.