Hi UncleAlex
Unlike accidents and dangerous occurrences where the "responsible person" for reporting under RIDDOR can usually make a decision on reportability quite quickly the process for reportable "diseases" (the very few that are listed) is highly reactive and dependent on a medical practitioner coming up with a "disgnosis" which is a defined term in RIDDOR:
“diagnosis” means a registered medical practitioner’s identification (in writing, where it pertains to an employee) of—
(a) new symptoms; or
(b) symptoms which have significantly worsened;
So, ignoring the issues of agency employment for a moment, if the first time a medical practitioner writes ona piece of paper that someone has work-related HAVS, then it is up to the responsible person to tell the enforcing authority about this, even if the symptoms might relate to work in the past.
However if this person is employed by an agency then the agency is their employer and hence the "responsible person" by virtue of Regulation 3.
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.
If HSE has done any research into how much underreporting there is under RIDDOR 2014, they don't appear to have made the results public.
However, under the previous iteration of RIDDOR, HSE estimated that about half of all reportable accidents to employees were NOT reported and that over 90% of reportable accidents to the self-employed were NOT reported at a time when RIDDOR made the self-employed responsible for telling the authorities about accidents to themselves.
So, you can see that this was dealt with in RIDDOR 2013 by taking the onus off a self-employed person to report an ACCIDENT, putting that duty on the person in control of the premises where an incident had occurred.
Not so easy to do the same when it came to reportable occupational DISEASES as it would be sometimes difficult to determine who was in control when an incident had happened as so often the disease might be the result of repeated exposures often for multiple clients of the self-employer person.
I wouldn't worry about the possiblity of HSE getting multiple reports of occupational diseases, as the base line numbers are very small when compared to accidents. If they wish to, HSE statisticians could identify the double counting but probably won't bother as they are probably aware that such incidents are UNDERreported to a much more significant level than any multiple reporting.
For the HSE statisticians the much bigger problem is working out the true levels of occupational ill health which they estimate via sources mostly other than RIDDOR.