This paper presents an investigation of the extent to which Heine’s (2003) mechanisms of grammaticalization—erosion (phonetic reduction), decategorialization (loss of morpho-syntactic properties), desemanticization (semantic bleaching) and extension (context expansion)—are evident in the variation of negative question tags in three varieties of British English spoken in Glasgow, Tyneside, and Salford. The study considers the variation in terms of three types of variant—full (e.g., isn’t it), reduced (e.g., int it), and coalesced (e.g., innit)—which each represent a stage in the erosion process. Quantitative variationist analysis of informal conversational data shows that erosion of negative tags occurs to different degrees in each of the three communities. The locality with the least tag erosion—Tyneside—displays particularly strong social stratification in the variation that suggests a change in progress led by younger men. However, there is little to no evidence of decategorialization in the negative tags, nor does variation in tag meaning correlate with phonetic form in a consistent manner. The results therefore suggest that erosion and desemanticization/extension do not occur in lockstep as these constructions grammaticalize, while decategorialization occurs at a later stage in the change.