Save as provided in regulations 7 and 9, in cases where works are initiated by an authority and an undertaker executes measures which need to be taken in respect of his apparatus because of those works, the authority shall pay to the undertaker a sum equal to the allowable costs incurred by the undertaker in taking those measures, provided that the authority shall pay only 82 per cent of the said allowable costs if the authority pays to the undertaker a sum equal to 75 per cent of the estimate of the said 82 per cent pursuant to paragraph (a) of regulation 9.