Those hiring out equipment for use at work are considered as suppliers under section 6 of the Health and Safety at Work Act and so have broad responsibilities for the safety of the products they hire out. This includes (so far as reasonably practicable) pre-hire testing and inspection to ensure continued safety, and the provision of information.
Although they need to look for obvious safety defects (in terms of the initial safety of CE-marked equipment), those hiring out equipment do not have to go into the detailed design of that equipment or enhance its safety beyond the requirements of the relevant product supply Directive - provided the product is CE marked, accompanied by user instructions and, where relevant, a Declaration of Conformity.
Those hiring out work equipment also have responsibilities under PUWER (and if lifting equipment LOLER), in so far as they exercise control over that equipment (for example, ensuring the thorough examination of lifting equipment and other routine inspections have taken place at the required intervals). It may be appropriate for the user to organise the periodic thorough examinations (which should be by written agreement, particularly for long-term hire). However, unless part of the hire agreement, those hiring out work equipment can't normally be responsible for the day-to-day and other pre-use safety checks which should be undertaken by the user.