Protect a Beneficiaries interest

The property was registered in the name of Mrs L , A and B as tenants in common. Mrs L passed away and X is her executor. Land Registry have however removed Mrs L from the title leaving A and B as the registered proprietors with a Form A restriction noted. Mrs L however left her share of the property to C which A and B are not happy about. How can we protect C - as it stands, A and B can sell the property without X knowledge, they will be able to overreach the Form A restriction currently on the title and effectively leave the country with the sale proceeds ? (We have tried an RX1 and a UN without success)