To my great relief, and I'm sure to yours as well, the aforementioned Weetabix disaster has been rectififed, as follows.
Amazingly, when I came to the final biscuit, one solitary, lonely, single biscuit, that I placed forlornly and sadly into the bowl and then, as normal, tipped the biscuit crumbs in on top, there were so, so many biscuit crumbs that there was no need, indeed no room, for a second biscuit. I was able to have a satisfying bowl without taking a single biscuit from a new packet.
Hurrah! I'm back to the correct scenario.
I am happy to bring this news to you.