It looks like the 'creative' writing is going to need to be quite formulaic given the time and the need for a consistent mark scheme.
It may even be that it is only marked for those children who are still 'in with a shot' after the short answer questions have been marked.
Hopefully the sample papers will give you an idea of how much time will be allocated to the writing but I would say that you would be wise to get DCs to practice writing short pieces that demonstrate as many skills as possible - getting basics correct with a few twiddles and fancy vocab thrown in on top
Be interesting to know whether they are bothered about the piece being complete or will be looking for evidence of sentence structure etc. as that is what some GS staff are complaining about (i.e. year 7s coming in are unable to write any thing comprehensible never mind writing an interesting story!)