Personal services relates to the IR35 tests HMRC can carry out to ascertain if someone is truly self employed contractor or a disguised employee. Ask yourself the following questions :   

Do you personally have to carry out the work?

Are you required to carry out the contract work yourself or can you substitute someone else? 

Do you have a substitution clause in your contract agreement?

Do you or the contract manager control the substitution?

If you fail to meet the criteria and have a tax investigation employer may be fined but also be required to settle any back pay due on Income Tax and National Insurance payments for what are considered “Employed” staff.  

Jon Norris of Crunch accounting explains in the video below.  

If in any doubt we strongly advise you to seek independent professional advice to ensure you don't get in to trouble with the HMRC who can impose fines and demand back payment of income tax and National Insurance contributions.