[peteo]

Can a person be left alone at their place of work?

There are no absolute restrictions on working alone;
it will depend on the findings of a risk assessment.
There are two main pieces of legislation that will apply:
The Health and Safety at Work etc Act 1974: Section 2 sets out a duty of care on employers to ensure the health, safety and welfare of their employees whilst they are at work.
The Management of Health and Safety at work
Regulations 1999:
Regulation 3 states that every employer shall make a suitable and sufficient assessment of -
·   the risks to the health and safety of his employees to which they are exposed whilst they are at work; and
·   the risks to the health and safety of persons not in his employment arising out of or in connection with the conduct by him of his undertaking
The HSE have produced the following publication: