fixed contract

Letting Agents

just a query my grandmother has been in a rented property for the past 18 years always been a good tenant paid on time property in mint condition etc... the property is managed through the local letting agents, over the past couple of years my grandmothers health has detoriated and she has struggled with dementia the letting agents were advised of this by several relatives and to communicate with us directly, it has now become time to put her into residential care as there is safeguarding issues. we have contacted the letting agents to give her months notice but they have advised they posted tenancy agreements to my grandmother in May and she signed for a fixed 12 month tenancy, nobody was made aware of this and we all assumed was periodic roll on contract. The letting agents are saying as she has signed a fixed tenancy she is responsible for the rent until the end of this period or until another tenant is found, with her residential costs this is going to be difficult financially. Our agruement is that she does not have mental capacity to enter into an agreement but they are arguing it has been countersigned by a witness in her home so agreement still stands, where do we stand with this? any advise would be greatly appreciated