CDM liablity when excavating

A company breaks ground in various ways by mechanical excavation or by pushing steel rods into the ground to obtain load bearing information for construction projects for clients.
But before any breaking ground work starts the clients checks their underground service drawings (electric, gas, water etc.) and then marks the area/s for breaking ground together with a CAT scan (Cable Avoidance Tool) before any breaking ground work starts.
In the event there is any injury (electric cable contact for example) who would be liable?
Reading from my NEBOSH construction certificate text book the Health and Safety at Work act says that the Employer’s common law duty of care is – The employer has a duty of care to each of his/her employees. This duty cannot be assigned to another person.
That suggests to me it’s the employer (the company that breaks the ground) to ensure employees safety and not the client.
However the CDM regulations tell me regulation 4 headed – Client duties for managing projects (again form my text book) says - Arrangements are suitable to ensure that construction work to be carried out so far as is reasonably practicable , without risk to the heath or safety of any person/s effected by the project.
I’m genuinely confused and I would be grateful for any information but please refer to the relevant regulation or source for your information so I can pin point the duties.
Thank you.