This methodology article presents a novel, mixed-methods framework designed to systematically analyse the political will and implementation gap undermining South Africa's Health Promotion Levy (HPL) on sugar-sweetened beverages (SSBs). The central research problem is the persistent failure to enact evidence-based adjustments to the levy, despite its proven public health efficacy, revealing a critical disconnect between policy intent and fiscal enforcement. To address this, the proposed methodology triangulates three analytical streams: a longitudinal policy tracing of parliamentary debates and fiscal reviews; a structured stakeholder influence analysis mapping industry versus public health actors; and a qualitative content analysis of media narratives. This integrated approach moves beyond singular explanations to dissect the multi-layered political economy constraining effective SSB taxation. The framework's application demonstrates that the implementation gap is not a technical failure but a consequence of sustained industry lobbying, shifting political priorities, and a discursive framing that pits economic concerns against health imperatives. Its significance lies in providing public health researchers and policymakers with a rigorous, replicable tool to diagnose and address political barriers to nutrition policy within African and similar contexts. The methodology underscores that achieving the intended outcomes of fiscal measures requires analytical tools that explicitly treat power and political will as central determinants of implementation success.