NIM02921 - Tips, gratuities and service charges: Gratuity disregard conditions: Condition 1
Paragraph 5(2) of Part 10 of Schedule 3 to the Social Security (Contributions)Regulations 2001
If an employer does not pay gratuities or payments in respect of gratuities either directly or indirectly to their employees and the payments do not comprise or represent sums previously paid to the employer, the payments can be disregarded from earnings. This is referred to throughout this manual as condition 1 of the gratuity disregard.
So, where an employee receives payments of gratuities direct from a customer and the employer is not involved in making the payments, there will be no NICs liability.