Retraction of /s/ to a more [ʃ]-like sound is a well-known sound change attested across many varieties of English for /stɹ/ words, e.g. street and strong. Despite recent sociophonetic interest in the variable, there remains disagreement over whether it represents a case of long-distance assimilation to /ɹ/ in these clusters or a two-step process involving local assimilation to an affricate derived from the sequence /tɹ/. In this paper, we investigate Manchester English and apply similar quantitative analysis to two contexts that are comparatively under-researched but allow us to tease apart the presence of an affricate and a rhotic: /stj/ as in student, which exhibits similar affrication of the /tj/ cluster in many varieties of British English, and /stʃ/ as in mischief. In an acoustic analysis conducted on a demographically-stratified corpus of over 115 sociolinguistic interviews, we track these three environments of /s/-retraction in apparent time and find that they change in parallel and behave in tandem with respect to the other factors conditioning variation in /s/-retraction. Based on these results, we argue that the triggering mechanisms of retraction are best modelled with direct reference to /t/-affrication and with /ɹ/ playing only an indirect, and not unique, role. Analysis of the whole sibilant space also reveals apparent-time change in the magnitude of the /s/–/ʃ/ contrast itself, highlighting the importance of contextualising this change with respect to the realisation of English sibilants more generally as these may be undergoing independent change.