quotations yes, singular, but not speechmarks
difference:
Psammead said, "hello." = speechmarks
the quote inside that text from earlier was 'yes dear'
but if you'd just said, "yes dear," than it would be double ones.
so:
Psammead said, "Chaos, me too. Sort of that dismissive 'yes dear' stock reply I generally give when DH starts whittering on about turning off lights."
but:
'British practice is normally to enclose quoted matter between single quotation marks, and to use double quotation marks for a quotation within a quotation... The order is often reversed in newspapers, and uniformly in US practice.'
because you're quoting what the passage said, you're not saying it.
It's quite hard to explain, isn't it!
eg:
Last year, I saw a website that was selling 'really good stuff'.
Monica said, "there's some really good stuff on this website."
see?
PS: Minnie - it's probably also down to "house style" - your company wants single quotation marks, even when they're speaking, others would want it done the correct way 