I have had a response from Cowshed regarding the missing serum. They've apologised and said they would be sending a replacement as a 'gesture of goodwill'.
I and many others relied on the website listing of contents before spending money, even on a discounted AC, due to more limited spending this year. It seemed pretty poor that the most expensive item listed in the contents blurb was the item that was swapped out ... for a nail file.
Will update when/if the serum arrives.