(5)The deduction to be allowed under sub-section (1) for any relevant tax year shall be—(a)an amount equal to one-tenth of the expenditure specified in sub-section (2) as reduced by the expenditure mentioned in sub-sections (3) and (4) (such one-tenth being herein referred to as the instalment); or(b)such amount as is sufficient to reduce to nil the income (as computed before making the deduction under this section) of that tax year arising from the commercial exploitation [whether or not such commercial exploitation is as a result of the operations or development referred to in sub-sections (2) and (3)] of any mine or other natural deposit of the mineral or any one or more of the minerals in a group of associated minerals under this section in respect of which the expenditure was incurred,whichever is less.
