The message

I was looking through the files for an official letter and started giggling. Kit said, "What is it?" I said, "I found a letter from the bank, and there's a message I don't recognise on the back in my handwriting."

The message says:

I'm a little teapot, brave and bold,
Leave the tea until it's cold.
When a fortnight's passed, I'm filled with mould.
Don't pretend you were not told.