After manifesting, if you go to the post office and it's closed, all bags/tags/labels/manifests become redundant, you cannot simply take them to the post office the next day. Everything has to be done again from the beginning, including entering all label data manually, and requesting a refund MANUALLY via an email for so you aren't charged for the manifested items that were never sent. Why is there not just a "unmanifest" button?
Royal Mail/Post Office happy to accept bags even if they are either a day early or a day late. We've done this many times with no problems... Royal Mail don't encourage this, as it just screws up their internal data.
Why can you not still take them?
Because the manifesting is complicated and involves more than just C+D. This is why they can't give us the prices until after it has been manifested.
You need to contact RM and find out where you take parcels after your local PO has closed - we have a RM depot near here we take them to after PO hours.