This is something that should be made clear in the contract between the agency and the host company however, if you go by the regs.... the employees employer, (assuming I guess the host is paying the agency and there is no other contractural arrangement in place).  In addition the responsibility for reporting does not imply liability for neglegence.  Also it may not be reportable unless the employee was actually engaged in the activity of loading or unloading the goods (see regulation 14 Restrictions on the application of regulations 4 to 10 (c) was engaged in work connected with the loading or unloading of any article or substance onto or off the vehicle at the time of the accident, or was injured or killed by the activities of another person who was so engaged; or

Responsible person

3.—(1) In these Regulations, the “responsible person” is—

(a)in relation to an injury, death or dangerous occurrence reportable under regulation 4, 5, 6 or 7 or recordable under regulation 12(1)(b) involving—

(i)an employee, that employee’s employer; or

(ii)a person not at work or a self-employed person, or in relation to any other dangerous occurrence, the person who by means of their carrying on any undertaking was in control of the premises where the reportable or recordable incident happened, at the time it happened; or

(b)in relation to a diagnosis reportable under regulation 8, 9 or 10 in respect of—

(i)an employee, that employee’s employer; or

(ii)a self-employed person, that self-employed person.

(2) Despite paragraph (1), in these Regulations the “responsible person” is—

(a)in relation to a mine, the manager of that mine;

(b)in relation to a closed tip, the owner of the mine with which that tip is associated;

(c)in relation to a quarry, the operator of that quarry;

(d)in relation to a dangerous occurrence—

(i)at a pipeline, the operator of that pipeline; or

(ii)at a well, the person appointed to organise and supervise the drilling of, and operations using, that well by any person granted a licence under section 3 of the Petroleum Act 1998(1), or where no such person is appointed, that licensee; or

(e)except in relation to a diagnosis reportable under regulation 8, 9 or 10—

(i)at an offshore installation, the duty holder for the purposes of the Offshore Installations and Pipeline Works (Management and Administration) Regulations 1995(2) (provided that for the purposes of this provision regulation 3(2)(c) of those Regulations is deemed not to apply); or

(ii)in relation to a diving project, the diving contractor.