I'm starting to think they made that letter up. When I spoke to them yesterday they didn't even mention any letter being sent to my address.
And I'm almost 100% positive we didn't leave out any of the paperwork. We spent two months going over it, and had friends go over it as well. We did forget to include two of our bank statements, but that was out of the 11 we had already included and we sent the missing two via recorded delivery with a letter explaining what they were. It says they received them less than a week after we had sent off the original application.
I received my biometrics letter so quickly, I guess I just naively assumed it would be an easy road because of that.