Pimmsoclock, we were a bit unsure about the marking scheme for creative writing too.
With marks of between 20 and 25 per creative writing, we assumed approx. 5 marks would be awarded for each of content/structure, grammar, vocab, spelling and sentence structure.
For content, dd aimed to show her writing was a relevant continuation, or followed the given title and had a beginning, middle and end/conclusion. For vocab, we encouraged dd to identify the atmospheric/emotional theme and use related vocab accordingly. For sentence structure, dd aimed to include a few 'tools' such as similies etc to demonstrate ability to use appropriately. Grammar aimed to show ability to capitalise, use colons, paragraphing etc.
It is a bit formulaic and only how we approached it. Encouraged her to write a quick plan on spare paper to ensure nothing was forgotten.