I’d say more of a legal minefield to have unpaid Marshall’s then to have paid marshalls
A business should sustain itself and will have the tax man being very attentative if they don’t appear to have the staff to run the business - smells of cash in hand and black markets
There’s no problem with dealing in cash - you just need accounts and records
The Health & Safety Executive, insurers and local council would be interested when something happens and there is no one looking after safety
You don’t need to have self employed staff to make it easier for the site. Establish whether PAYE, pensions etc apply and set the wage
The site may pay minimum wage and not take tax deductions if the people are below the thresholds.
If the marshal only has a part time / zero hours contract single job then happy days, if they have a second or third job then it gets decided to pay tax from pay or the person has to declare their own returns