The Guardian style book agrees with Xenia! Para 5.
apostrophes
used to indicate a missing letter or letters (can't, we'd) or a possessive (David's book).
Don't let anyone tell you that apostrophes don't matter and we would be better off without them. Consider these four phrases, each of which means something different:
my sister's friend's books (refers to one sister and her friend).
my sister's friends' books (one sister with lots of friends).
my sisters' friend's books (more than one sister, and their friend).
my sisters' friends' books (more than one sister, and their friends).
The possessive in words and names ending in S normally takes an apostrophe followed by a second S (Jones's, James's), but be guided by pronunciation and use the plural apostrophe where it helps: Mephistopheles', Waters', Hedges' rather than Mephistopheles's, Waters's, Hedges's.
Plural nouns that do not end in S take an apostrophe and S in the possessive: children's games, old folk's home, people's republic, etc.
Phrases such as butcher's knife, collector's item, cow's milk, goat's cheese, pig's blood, hangman's noose, writer's cramp, etc are treated as singular.
Use apostrophes in phrases such as two days' time, 12 years' imprisonment and six weeks' holiday, where the time period (two days) modifies a noun (time), but not in nine months pregnant or three weeks old, where the time period is adverbial (modifying an adjective such as pregnant or old) ? if in doubt, test with a singular such as one day's time, one month pregnant.
Some shops use an apostrophe, wrongly, to indicate a plural ("pea's"), but will generally omit the apostrophe when one is actually required ("new seasons asparagus"), a phenomenon sometimes referred to as the greengrocer's (or grocer's) apostrophe. Try to avoid this