If a charity hires someone not as an employee who has their own personal service company (PSC) which invoices the charity, there is currently no risk to the charity of HMRC saying that person should have been on the payroll as an employee. However the PSC may need to account for tax and NI as if they had been an employee under an HMRC rule called IR35.
In 2017, new off payroll working rules were introduced for public sector workers. Under these rules the responsibility for operating IR35 shifted from the PSC to the public sector engager. HMRC is now considering extending these rules to the private sector (which would include charities). To aid engagers HMRC has also updated its employment status indicator (ESI) tool, it is now called the Check Employment Status for Tax service (CEST). This can be used if you have any individual working for you on a consultancy basis and you are unsure if they should actually be treated as an employee.
source sayervincent may 2018