If, on the taxation of a bill of costs, the amount of the professional charges contained in the bill is reduced by a sixth or more –

(a) costs are not allowed to the solicitor for preparing of the bill, attending the taxation or any matter relating to the taxation, unless the taxing officer otherwise determines; and

(b) if costs are disallowed under paragraph (a) , the taxing officer may allow to the party chargeable by the bill any costs arising from any necessary preparation for, and the attendance on, the taxation, as the taxing officer thinks proper; and

(c) if the taxing officer makes an allowance under paragraph (b)  –

(i) the amount of that allowance is to be set off against any costs the party is liable to pay; and

(ii) the taxing officer is to certify in the certificate accordingly.