I would challenge that consultant about the need for that signage, i.e. his assessment that it is essential requirement.

The guidance accompanying Regulation 7(3), (referring to L 138 i.e The ACoP & Guidance to DSEAR) "Marking of areas containing explosive atmospheres" , and specifically paras 249 & 250 are relevant to your situation.

On one hand, para 249 states that where a sign is necessary, it should be positioned at the points of entry to the place where the explosive atmosphere may exist. Where an explosive atmosphere may occur within a clearly defined area, the appropriate point or points of entry will be easy to determine. However, in other cases explosive atmospheres may occur in places that do not have obvious entry points, for example in multiple places around a large open-air chemical plant. Such places are often already identified in other ways, for example by painted lines on the ground to indicate hazard zones. Signs may, therefore, not always be appropriate but the risk area must be identified and its presence communicated by some other means.

On the other hand, para 250 states that where a sign or signs are considered necessary to further reduce risk, in addition to other measures, it may be more appropriate in some cases just to mark points of entry to the workplace as a whole, rather than the numerous individual locations within the site as well, if all the special precautions apply throughout the entire site.

The key here is whether this was simply a back covering aspect by the consultant or has para 250 been applied properly. Only by questioning the reasoning behind this can it be known. My view, without knowing details is that it serves no or very little purpose