13.  For the sums specified in the Third Schedule, there shall be substituted the following sums—

Exceeding £50 but not exceeding £56	£1
Exceeding £56 but not exceeding £60	£2
Exceeding £60 but not exceeding £64	£3
Exceeding £64 but not exceeding £68	£4
Exceeding £68 but not exceeding £72	£5
Exceeding £72 but not exceeding £76	£6
Exceeding £76 but not exceeding £80	£7

,

and the weekly instalment of contribution shall be increased by £1 for each £4 or part of £4 by which average weekly disposable income exceeds £80.