Victor - a quick thought: did your client already know the 2 visitors to be HSE inspectors? If not, just how certain was it that the visitors were genuine?!!
As a general piece of advice, surely most or perhaps all enforcement officials, e.g. from HSE, Environment Agency, HMRC, etc., have to prove who they are if requested to do so. Therefore, though there is a reluctance to challenge unknown people who arrive and simply state that they are, e.g. an inspector from HSE, surely there's no harm in politely asking for proof. Those who feel uncomfortable doing this could add that it's standard procedure for every new visitor.
Every HSE inspector has a warrant which contains their full name plus mugshot photo embossed with a crest, and should carry it on their person to produce it if requested to do so. (Mind you, from recollection of my decade with HSE I was rarely asked for identification. This tended to be at the premises of small firms visited during 'on spec' night inspections as some of their people were reluctant to believe that inspectors ventured out at night. By contrast, some larger firms where I would have expected a higher degree of security seemed surprisingly lax and took me at my word!)
Surely enforcement people shouldn't be offended at being asked for identification. If they don't like it or refuse, it could stem from several or more possibilities: They are i) being pompous, ii) embarrassed to say that they have forgotten to carry their identification with them or iii) not genuine! It's an offence to impersonate an HSE inspector and presumably there are laws about impersonating other types of enforcement officers.
With matters like commercial secrecy and people thinking of new ways to defraud, especially if the HSE charging regime goes ahead, it makes sense to check that enforcers are who they say they are.
One example of fraudulent audacity, though not involving enforcers, was shown on BBC TV's "Motorway Cops" last night. Thieves arrived with a tanker at a filling station which had closed for the night, somehow opened the hatches over the underground fuel tanks and started pumping diesel up into their tanker. Most passers-by would pay little or no attention and just assume that a delivery was being made. Fortunately, the filling station's CCTV system was being monitored remotely and the police were notified. No doubt other forum users who saw the programme noted that when the thieves scarpered as the police suddenly arrived, they left their pump running. As a result the tanker compartment became full and started to overflow and spread over the forecourt and down the adjacent road. Fortunately the fuel being stolen was diesel and not petrol which could easily have been ignited and caused a major explosion and fire!
A few apologies for deviating somewhat from topic. Hopefully forum users, retailers, fuel distributors and security firms involved with vehicle filling stations will be aware of the various risks from this type of theft and therefore have appropriate counter-measures in place.