I just called UKVI where a very sullen young man informed me that -- and this is a direct quote -- "it doesn't matter if the passport expires while the application is being processed, because the visa is now issued as a separate biometric card". So according to him, as long as the passport is current when we submit, we should be OK.
Does this sound too good to be true to anyone else? Might still go the checking service route as if it does come back for want of a current passport, we'd have it already!