Depends Who identified this as high risk. If it was an asbestos surveyor and there was an immediate risk of exposure to employees then he should have recomended that the area be immediatley sealed off. Could you specify the type of ACM, its condition and its location. Also you cannot report to GP`s but if you feel there has been exposure you should provide suitable advice and guidance to the affected employees to enable them to make that decision. I believe this would also be reportable under RIDDOR. "Hazardous escapes of substances" "27 The unintentional release or escape of any substance which could cause personal injury to any person other than through the combustion of flammable liquids or gases. The substances covered by this definition may be in any form: liquid, solid (eg powder), gaseous or vapour and may include, eg: substances which may be hazardous to health (eg asbestos, phosgene, toluene diisocyanate)"

If there was debris found on the floor then it is pretty certain that there was a degree of contamination especially given the friability of the material. As such it is highly likely that exposure has occurred. With regards to providing information to staff who may have been affected, try contacting EMAS (employment medical advisory service) who used to have a free leaflet that you can give to people who have been subject to asbestos exposure. The leaflet advised them on what they should do with regards to informing their GP etc and also was very proportional in advising them on the (minimal) risk of a one off asbestos exposure. Clearly from what you describe though there is the potential for this material to have been deteriorating and releasing fibres for some time . As to whether you take this further and report it to the HSE ? Legally I would say you should as described previously under RIDDOR. Personally speaking I think you would have to. Especially if you are going to advise people they have been exposed, because if you havnt they will. The people responsible for this building have clearly known about this for some time and have ignored the asbestos surveys findings and knowingly exposed people to this so they should face the consequences. IMO