If any sum is paid by the county council of Essex, Hertfordshire, Kent or Surrey as compensation in respect of loss of employment, or loss or diminution of emoluments, which is attributable to any provision of the Act or of any instrument made under the Act, the Greater London Council shall repay such proportion of such sum as may be agreed between them and such county council, or, failing such agreement, as may be determined by an arbitrator appointed by agreement between them and such county council or, in default of agreement, by the Minister.