If you work as an employee for someone, a contract
automatically comes into effect between the two of you, even if no written document ever exists. (An employer has a legal obligation to provide an employee with a 'written statement of employment particulars' within two months of them
commencing work. Even if the employee leaves almost immediately that legal obligation still exists, but failure to comply with that obligation does not negate the contractual arrangement which automatically came into place at the start of the employment).
If you work just a single hour for someone (as an employee, rather than as self-employed) you're still entitled to holiday pay.
If your staff had worked for you for a full year they would have been entitled to 5.6 x 16 hours holiday pay. Since they worked 1/13 of a year, they're actually entitled to 5.6 x 16/13 hours pay = 6.892 hours.
If you want to check my figures, start here:
https://www.gov.uk/calculate-your-holiday-entitlement
The accrued holiday pay should have been added to your staff's final week's pay, with PAYE and National Insurance (if relevant) being calculated upon the aggregate payment:
http://www.hmrc.gov.uk/manuals/nimmanual/nim09170.htm