Thanks guys for the comments,
have been doing some study on the matter, and the whole thing is quite interesting. The decibel ratings that are advertised on headset data are more than likely far in excess of what you will actually get.
For example, a well known headset (that a lot of you are probably using) quotes 25db of noise reduction and in independent test only reached between 5 - 11db noise reduction depending on the environment. Its for this reason that one of the best headsets on the market do not quote any level of attenuation because of the wide range of levels that will be achieved depending on the situation (head shape, noise freq's, sunglasses etc).
The other interesting fact to come out of this is that using foam plugs under you existing headset in a crewed environment, while giving you pretty good attenuation may be making your crew deaf if they aren't using plugs as well because the intercom is turned up so loud to compensate!
ANR seems to be the way to go, but once again has it's down falls. Not all ANR will attenuate the freq's required in your operation so care should be taken when selecting your next ANR headset.
FC