Footnotes

• Contributors RMB conceived and wrote the paper.

• Funding The author has not declared a specific grant for this research from any funding agency in the public, commercial or not-for-profit sectors.

• Map disclaimer The inclusion of any map (including the depiction of any boundaries therein), or of any geographic or locational reference, does not imply the expression of any opinion whatsoever on the part of BMJ concerning the legal status of any country, territory, jurisdiction or area or of its authorities. Any such expression remains solely that of the relevant source and is not endorsed by BMJ. Maps are provided without any warranty of any kind, either express or implied.

• Competing interests None declared.

• Provenance and peer review Provenance and peer review. Not commissioned. Externally peer reviewed by James McDonald, Edinburgh, UK and Colin Mumford, Edinburgh, UK.