1 Newcastle University, Newcastle Upon Tyne, UK; South Tyneside District Hospital, South Shields, Tyne and Wear, UK. Electronic address: colin.rees@stft.nhs.uk.
2 South Tyneside District Hospital, South Shields, Tyne and Wear, UK.
3 Newcastle University, Newcastle Upon Tyne, UK; Freeman Hospital, Newcastle Upon Tyne, UK.