https://www.acas.org.uk/contact
ACAS is a free to use government sponsored facility to help disputes between employees and employers regarding wages.
it is a friendly and helpful service and available to all.
You can call on 0300 123 1150 and they are open usual office hours from Monday at 9am.
It is also possible to have someone deal with ACAS on your behalf, with permission.
This facility is not relevant for self-employed where the small claims court facility at Sheriff Courts in Scotland is the main option if all else fails, although, personally I have found the “shirker of the week” route, albeit perhaps actionable, can get results.