This is taken from the CAA website:
A reportable occurrence in relation to an aircraft means any incident which endangers or which, if not corrected, would endanger an aircraft, its occupants or any other person.
A report should also be submitted on any occurrence which involves, for example, a defective condition or unsatisfactory behaviour or procedure which did not immediately endanger the aircraft but which, if allowed to continue uncorrected, or if repeated in different, but likely, circumstances, would create a hazard.
The difference being that it doesn't actually have to occur.
MOR reporting
There we've trawl the internet for you, saves you doing it I suppose
