(1) Costs not reasonably incurred or not of reasonable amount may nevertheless be allowed to a solicitor against a client if—
(a) the costs were incurred with the authority of or the amount was authorised by the client; and
(b) before the costs were incurred the solicitor expressly warned the client that the costs might not be allowed on a taxation of costs as between party and party.
(2) An authority for the purpose of this Rule may be express or implied.
(3) Where the client is a person under disability, references to the client in paragraph (1) include references to the litigation guardian of the client.