Our year-long collaborative consultancy project aimed to understand and ultimately improve client opt-in rates in an NHS Diabetes Education Service, following adjustments to service delivery during the Covid-19 pandemic. This case study illustrates the importance of enquiry in consultancy, which enabled our client to understand the stories behind the reducing opt-in rates and shaped the co-development of effective patient-led interventions. This project describes enquiry focused on service non-attenders, a relatively under-heard voice whose insights were key to the success of this project. In this chapter we take a broad view of enquiry throughout the life cycle of a consultancy project with a focus on effective client engagement. The project led to sustainable service transformation and improved opt-in rates, alongside system-wide action and commitment to address health literacy barriers across the local Integrated Care System. Alongside a deep dive into the ‘how’ and ‘why’ of an enquiry approach to consultancy, further consultancy themes discussed in this chapter include pros and cons of internal consultancy, the realities of consultancy in an NHS Service context, and the practicalities of working with supervisees to deliver consultancy.