I needed a new passport and I knew it was going to be complicated - my old one was expired and I've managed to lose it (albeit after it was out of date), AND I needed to change the name on it. I couldn't work out what I needed to do from the stuff the passport office sent me (it's allegedly passed by the Plain English people but I have an English degree and couldn't find any definitive answer about what I needed to include in the application) so I took my stuff to the PO and paid £7 for the Check and Send thing.
A nice lady from the Passport Office has just phoned me up and told me that the photos should have been countersigned (you don't necessarily need to do this for a renewal, which is WHY I TOOK IT TO THE POST OFFICE) and they might also need my birth certificate. So not only was their own advice obviously pretty poor - I sat and read through all the documents more than once without realising any of this - but I've paid £7 for nothing.