i leave it off, as it's a waste of battery, and just put *0# at the start of every message I want a delivery report for

Try it, send your girl a text with:

*0#Hello


Of course, because youre texting a girl and their phones are glued to their hands/checked every 37 milliseconds, youll probably get a reply before the delivery report, but a text comes through from o2 saying:

Message 1234 to NUMBER submitted TIME delivered TIME text:Hello...