Here is the relevant provision in the Solicitors Act 1974 which forbids contingency fees and which makes the bank’s contract with its solicitors unlawful:
“59 Contentious business agreements….
(2) Nothing in this section or in sections 60 to 63 shall give validity to—
(a) any purchase by a solicitor of the interest, or any part of the interest, of his client in any action, suit or other contentious proceeding; or
(b) any agreement by which a solicitor retained or employed to prosecute any action, suit or other contentious proceeding, stipulates for payment only in the event of success in that action, suit or proceeding; ….“