Matt actually spotted it. Although I'm usually really organised (making a list, checking it twice, etc), I would never have checked it as I know it's not 10 years old. When he told me, I didn't believe him at first, but it turns out it's a replacement of one issued in 1992.
We've been through the whole 'OMG - imagine if we'd found out at check-in' scenario about a thousand times and we've told our friends to check theirs too.
I'm not overly worried about getting a new one through in time (The Passport Agency's website indicates 2 weeks at the moment), although I <i>will</i> pay the £4 to have it checked in the circumstances - not something I'd normally bother with.