The law of negligence means any negligent act or which falls short of a standard to be expected of "the reasonable man"

What is the definition of “reasonable?” This appears to be a grey area as with much of U.K. law, and we suggest you speak to your solicitor to ascertain case histories

A famous case once ruled that a doctor is not in breach of the duty of care "if he has acted in accordance with a practice accepted as proper by a responsible body of medical men skilled in that particular art". 

In plain English this means that a judge will hear evidence from experts in the appropriate speciality and must decide whether the actions of the doctor/midwife/nurse etc. were proper. 

Often there are several acceptable ways of doing something and compliance and this may mean that there is no breach of duty of care. 

Naturally experts often disagree over these issues and the judge must decide whose evidence is to be preferred.