It’s a simple matter of having an accurate and auditable system of recording staff hours and when they are actually on-site.
Without an auditable record, how do you demonstrate an effective system of financial control when resolving to pay wages? How would you demonstrate due diligence and safe systems of work to HSE in the case of a workplace reportable accident?
It doesn’t ‘have’ to be a clocking system, but there does need to be an effective system