Miss Mills, 39, employed Mishcon de Reya for two years in the hope they would secure her a substantial chunk of Sir Paul's £825million fortune.
She is thought to have received only a relatively small interim payment.
This has left her so short of money that she plans to represent herself in a five-day High Court battle next week.
This will give her the opportunity to grill the 65-year-old Beatle personally.
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