Magistrates' Courts Act 1980

2 (1) Where the amount due at the time imprisonment is imposed is so much of a sum adjudged to be paid by a summary conviction as remains due after part payment, then, subject to sub-paragraph (2) below, the maximum period applicable to the amount shall be the period applicable to the whole sum reduced by such number of days as bears to the total number of days therein the same proportion as the part paid bears to the whole sum.

(2) In calculating the reduction required under sub-paragraph (1) above any fraction of a day shall be left out of account and the maximum period shall not be reduced to less than 5 days.