http://orcid.org/0000-0002-2776-4014Hazel Squires1, http://orcid.org/0000-0002-1701-0577Suzanne Mason1, http://orcid.org/0000-0002-4484-7401Colin O’Keeffe1, Susan Croft1, George Millington2 Correspondence to Dr Hazel Squires, Sheffield Centre for Health and Related Research, University of Sheffield, Sheffield S1 4DA, UK; h.squires@sheffield.ac.uk