Thanks again, Les. I see it now. Wouldn't have recognised 'm' as a proper abbreviation for 'middle'. Not in my Chamnbers or Oxford dictionaries but perhaps somewhere else. When setting, I never use an abbreviation unless it's listed in Chambers D.
Agreed it's a bit tortuous but it's implied in the abbreviations am and pm, taking m as meridiem (Latin noon) then checking out noon in Chambers as middle.
Les