Your line manager would be the obvious first port of call about an underpayment of salary incident, such as this. Either they can act on your behalf or you can get in touch with the payroll office yourself, to obtain an explanation (preferably in writing) of how they calculated this deduction.
Ideally, the act of drafting their reply to you will make them see the error for themselves and the adjustment could be in your next month's pay packet. If not, then at least you would have their reasoning on record before you take things further.
Expect to see calculations such as (for example: sick day on the 10th day of a 31 day month)
9 days at [annual full time salary] x 1/12 x 39/52 = £x.xx
1 days at [sick pay rate of 0.00] x 1/12 x 39/52 = £0.00
21 days at [annual full time salary] x 1/12 x 39/52 = £y.yy
I imagine their computer system limits them to something more like this...
[annual full time salary] x 1/12 x 39/52 = 1 month's pro-rata'd salary
deductions:
1 day @ [full time salary/365] = £z.zz
Although [salary/365] is not necessarily the way any/all employers would assess how much 1 day's salary actually amounts to. Some might refine it according to whether there are 28/29/30/31 days in the month in question.
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I just re-read your question and noticed you mention hourly rate. My answer may be unhelpful but I don't want to scrap what I've written so far.