You don't need Nick to provide that information - it is 1:100,000 flight hours. If you have two engines, the probability of one failing is 2:100,000 flight hours (two engines twice the exposure). The effect of failure is nil when in PC1 (or nil when above 200ft in PC2); or a forced landing when in PC3 (or when below 200ft in PC2).
The probability of both failing from independent causes is the probability of the first times the probability of the second - i.e. 1:10,000,000,000.
Acceptable risk is generally decided by society - on the basis of what is unacceptable risk. Risk is quantitatively stated in some rules - FAR/CS 29.1309 being the most well known (1:1,000,000,000). Acceptable risk is conditioned by the number of passengers exposed; that is why operational rules vary with the number of passengers carried.
None of what has been discussed is of the CAA's making; most of the rules are now provided by/for Europe - previously the JAA and now EASA. The basis for the 2002 deadline was to provide an incentive for improvement of landing sites at hospitals; no one wanted new helipads (in cities) to be too small or so badly sited that they would not be able to be used in PC1 (and it was size and location not helicopter performance that was the driver). At the time of the provision of the alleviation, 2002 was in the future.
Jim