So much of any sum for which provision for payment is made under this Agreement as is unpaid immediately after the date on which it falls due shall accrue interest from that date at 3% above the base lending rate from time to time of Barclays Bank plc:

Provided that if that rate shall at any time cease to exist or be ascertainable the Council shall substitute for it the base lending rate of such of the London Clearing Banks as it shall prescribe in writing.