The goal of this observational study is to learn about swallowing difficulties (dysphagia) in patients living with myotonic dystrophy type 1 (DM1). The main questions it aims to answer are: * whether the size and structure of the muscles involved in swallowing differ to those without the disease * how the size and structure of muscles may associate with swallowing function and swallowing symptoms in this group. Participants will undergo a range of tests including: * Ultrasound (US) assessment of the muscles involved in swallowing * An x-ray swallowing study (known as videofluoroscopy) * Assessment of swallowing symptoms, including questionnaires * Assessments of mobility, activity and breathing * Assessments of quality of life and wellbeing