I had a problem recently getting paid. After getting pushed around between various different departments and no one taking responsibility, I spoke to acas for advice.
I was told the first step was to send an email to the line manager who I worked for and was responsible for authorising my time sheet and also copied into payroll and the HR manager. The email set out the hours worked, how much was owed.
I was also told to include a paragraph about the financial impact of not getting paid (I really resented that, it should not matter if I have 3 kids to support over the summer or intended to use the money to take a bath in Moët).
The acas advisor also said to give them a reasonable length of time to respond, failing which further advise about industrial tribunal will be sought from acas.
I gave them 5 days notice and the money finally came through a total of 3 weeks late.
It seems I was caught between departments merging and the powers that be hadn't thought or cared about how lowly bank workers on the front line would actually receive payment.
Thankfully I have a new job starting in September, so don't need to worry about needing the audacity to ask for payment for work done.
Good luck op. Send an email today and ask acas if they still don't pay you the money owed