Leaver Pay calculations

Hi, I am very confused by the salary calculations done by my employer, and I believe there may be a mistake. My employment was terminated on June 21st, and I was paid for 15 days at a daily rate calculated from my annual salary (£45,000). They paid me 15 x £173.08 = £2596.20. Since I am on a monthly pay schedule, I believe I should have been paid £3750 - £865.40 (173.08 x 5) = £2884.60. This means I feel I have been underpaid by £288.40. All other months, I have been paid the monthly rate of £3750.00 and not a daily rate. When I raised this issue with my employer, they informed me that the daily rate is the correct method for calculating the salary when leaving the company. Could you please advise? Thank you.

Hi Jalakm,
This is not a tax query.
HMRC would not be able to offer any advice other than to contact ACAS.
Here are their contact details:                                  
Acas
Thank you.