It very much depends upon how much was released and the possible exposure - it could be reportable as a Dangerous Occurrence (release of a substance) - check out the guidance in L73. But with all RIDDOR's if in doubt always report!!!!

The method of "decontamination" will be determined entirely by the likely contamination! I suggest you get an analyst in to check. Not uncommon for fibres to be in the substrate holding the tiles to the floor also. Risk Assessment will cover all this of course. Isn't hindsight wonderful! ;-)

As has already been stated by my learned peer Graham; it's most unlikely that any asbestos exposure would have occurred because of the encapsulation of the material and what is needed to release a fibre where a member of the public was present [they should not have been present in the first place!] Additionally there is probably no way of confirming if an exposure has occurred from that particular event as asbestos is all over the place in the general environment that we walk about in so a member of the public, especially anybody over 40, has already been exposed e.g. Brake pads and clutches of vehicles on the public highway is one example! And I would read RIDDOR very carefully and follow through to the other associated guidance as referenced especially COSHH and CHIP and make a decision thereafter as there are certain definitions that need clarifying before you report and most if not all of the reporting parameters are for biological releases/exposure only and as we all know asbestos is not technicially a biological form Would I report? I would need more info about the specific event before I make a comment but generally if no significant release was possible therefore it could not have happened it would point me towards a non reporting situation but again read COSHH and CHIP