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Contributors There are three contributors and none have conflicted interests
Competing interests None declared.
Provenance and peer review Commissioned; externally peer reviewed. This paper was reviewed by Colin Mumford, Edinburgh, UK, and Tom Hughes, Cardiff, UK.
Data sharing statement Additional unpublished data mainly concern Dr Howlett’s unpublished data he is still working on.
Correction notice This paper has been amended since it was published Online First. Owing to a scripting error, some of the publisher names in the references were replaced with ‘BMJ Publishing Group'. This only affected the full text version, not the PDF. We have since corrected these errors and the correct publishers have been inserted into the references.
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