If the rent for premises is not worked out under section 52 (Market rent—rent reviews, options and renewals) or 53 (Valuation to work out market rent) before the extension of the lease is to commence or a rent review is due (the "relevant date")—
(a) the rent continues at the rate charged immediately before the relevant date until the rent has been worked out; and
(b) once the rent has been worked out and set, the party who owes the other party the difference between the rent paid and the rent that should have been paid must pay the difference to the party owed within 30 days after the rent is set.