Go to page 36 of Corgi's 1997 print of Soul Music where Glod, Lias and Imp are ordering food.Imp's accent is from Llamedos, where all "L"'s are pronounced "ll". Lias keeps insisting on buying coke, and Imp orders some "Hard boilled eggs". When he repeats his request, the double-l pronunciation is gone.
This isn't actually a slip-up; Terry Pratchett gradually fades the Llamedos accent out of Imp's pronunciations as the character spends time in the city (over the course of the book).