This IR138 tax leaflet was a useful but not authoritative guide to people living or moving abroad. The leaflet has been discontinued so here is a link to the old version:
https://webarchive.nationalarchives.gov.uk/20060715145138/http://www.hmrc.gov.uk/pdfs/ir138.htm
If you have an old copy of the .pdf leaflet please let me know so I can put a copy here on this website for others to read.