HSE enforces pesticides legislation under an Agency Agreement made between Ministers and the Health and Safety Commission (HSC).
In those premises for which HSE is the enforcing authority, Inspectors are authorised to enforce;
the Food and Environment Protection Act 1985 (FEPA); and
the Control of Pesticides Regulations 1986 (COPR).
the Plant Protection Regulations 1995
This means that, although pesticides legislation applies to the advertisement, sale, supply, storage and use of pesticides, HSE's enforcement role is restricted to storage and use in connection with work activities in premises where HSE has an enforcement role.
HSE is the “default” enforcer of health and safety legislation. If the activity is peripatetic, as it often is with pesticide use, HSE would enforce the above legislation.
This legislation talks in terms of ‘Authorised Officers’: most HSE inspectors are such officers and the authorisation is contained within their warrant.