(1) Section 284 of the Principal Act is amended -

(a) in subsection (2) -

(i) in paragraph (a), by substituting "paragraphs (aa) and (ab)" for "paragraph (aa)",

(ii) by inserting the following after paragraph (aa):

"(ab) Where for any chargeable period ending on or after 1 January 2002 a wear and tear allowance would be due to be made to a person in respect of machinery or plant in accordance with paragraph (a), the person may elect that the amount of the wear and tear allowance to be made for that chargeable period and any subsequent chargeable period in respect of each and every item of the machinery or plant concerned shall, subject to subsection (4), instead of being the amount referred to in paragraph (a), be an amount equal to -

(i) where, apart from this paragraph, the allowance would be made in accordance with paragraph (a) (i), 20 per cent of the amount of the capital expenditure incurred on the provision of that machinery or plant which is still unallowed as at the commencement of the first-mentioned chargeable period, and

(ii) where, apart from this paragraph, the allowance would be made in accordance with paragraph (a) (ii), 20 per cent of the value of that machinery or plant at the commencement of the first-mentioned chargeable period.