Personal Representative losing capacity

We have a case where an Attorney wishes to sign a contract for sale of property on behalf of the client in client’s capacity as executor and Personal Representative. The Grant was issued several years ago to client when she was capable, but the client has now lost capacity. The purchaser’s solicitors are insisting Attorney has no authority to sign on behalf of the incapable executor as the Grant was issued in the name of the client.

Does anyone have any experience of this or know whether there is an alternative to having to apply for a new Grant de bonis non or Double Grant (where a co-executor had power reserved)?

My direct email is

Many thanks in advance