Footnotes

• Funding NJS is supported by an NIHR Senior Investigator award and OJA by an NIHR Clinician Scientist fellowship award (grant number NIHR-CS-012-002). NJS and JCH are partially funded by the Great Ormond Street Hospital Children's Charity and the NIHR Biomedical Research Centre at Great Ormond Street Hospital.

• Disclaimer The views expressed are those of the authors and not necessarily those of the NHS, the NIHR or the Department of Health.

• Competing interests None declared.

• Ethics approval Local Research Ethics Committee.

• Provenance and peer review Commissioned; externally peer reviewed.