Salary arrears are illegal and can lead to big business problems: administrative fines, interest accrued on the delayed amounts, administrative and criminal liability for the director. But employees need to take care of protecting their interests themselves.
The law protects only employees with whom an employment contract is signed and only that part of their salary specified in this contract.
On this issue, you are better off contacting a lawyer at bookersandbolton.co.uk, who specializes in employment issues.