1S—Subject to paragraphs 4 and 5, the local authority shall forthwith demand the repayment to them by the owner for the time being of the house of the whole amount of any sums paid by the authority by way of improvement grant in respect of the expenses incurred for the purpose of the execution of those works together with interest thereon for the period from the date of payment of the grant, or where the grant was paid in instalments, from the date of payment of the final settlement of the balance by the authority to the date of repayment to the authority.