Tony Barrett's newsletter dated January 1993, which covers the previous 12 months basically states that Underworld 'has a new member, Darren Emerson' and they he and Rick were (mainly) responsible for the Lemon Interupt releases & Steppin Razor remixes in 1992.
The previous underworld contacts dated June 1991 doesn't mention Emerson (and in fact mentions UW working on a new album with Rupert Hine, in Spain) - so its basically betwen June 1991 & January 1993 when Emerson joined, although based upon the rough release schedules of the Lemon Interupt singles / Remixes by Steppin Razor, its likely to have been later 1991 / early 1992 when Darren 'officially' joined. An ID Magazine article dated October 1992 mentions Darren. I am ploughing through all the music press articles i have to see the first mention of Mr. DE, although i'm not sure all are accurate!
will report any further info when i find it....
here's the 1993 January Underworld contacts for
ref
big screen matt