Want of capacity — insane delusion

A 70-year-old woman in England has a fixed and irrational delusion that her only daughter is conspiring to poison her. She otherwise understands what a will does, the broad extent of her property and who her family members are. Because of that delusion she signs a will leaving everything to a cousin and excluding her daughter; without the delusion she would have left the estate to her daughter. The will is duly executed and she knows and approves its written terms. Nobody has pressured or deceived her. After her death, the daughter challenges her testamentary capacity. Is the challenge likely to succeed?