OBJECTIVES: The SPELL trial is a large-scale, multicentre, two arm, parallel group, superiority randomised controlled trial assessing the clinical effectiveness of an individually tailored dynamic stretching exercise programme compared to usual care in ambulant children with spastic cerebral palsy (CP). Here we describe the development, implementation and details of the physiotherapy-led interventions. METHODS: Medical Research Council guidance for developing complex interventions were used, taking into account clinical guidelines, expert and patient opinion, research evidence, current practice variation, and deliverability. A stakeholder meeting of 26 experts, clinicians, researchers, and patient representatives was used to design key components of the interventions. The interventions were designed to be deliverable across the UK's National Health Service, and this was tested during a six-month internal pilot. RESULTS: A child-specific dynamic stretching exercise programme was developed consisting of 6 sessions, overseen by a physiotherapist, over 16 weeks. Participants are asked to complete the dynamic stretching exercises 4-5 times per week. The programme incorporates behaviour change strategies designed to support long-term exercise adherence and is hosted on a fun web-based app, including exercise videos. Follow up sessions offer the opportunity for progression, supervised practice and feedback. The usual care comparator consists of one face-to-face session with a physiotherapist and content includes core, standardised components, alongside continuation of their normal physiotherapy care. CONCLUSION: Despite complexities and lack of good quality evidence, it was possible to develop, a widely accepted, well-defined dynamic stretching exercise programme. The SPELL trial will assess the effect of the dynamic stretching exercise programme on functional mobility in ambulant children with CP compared with usual care. Dissemination is anticipated in 2027. TRIAL REGISTRATION NUMBER: ISRCTN 15808719. CONTRIBUTION OF PAPER.