We have had a chat with DS's class teacher, and have started looking at the sample papers. The teacher has gone through the process with her own children so has both a professional and a personal opinion! She had one child tutored but the other didn't need it. Both got into their chosen school.
I would highly recommend starting with the person who knows your child best, academically speaking, before launching into tutoring which might not be needed.
In DS's case she is confident that there won't be any gaps in his knowledge or understanding that will need filling, which is where a tutor would be helpful, so it will simply be a matter of practising the kind of tasks, practising managing the answersheet, and eventually practising the discipline of concentrating on a single paper for 50 minutes without stopping, and of managing those fifty minutes to ensure that every question is answered.
There is a lot of creative writing in the normal Y5 curriculum, so our sons should be well prepared for it, as well as being used to including constructions "have I used direct speech? have I used adverbials?" and using the correct tone and formality (eg diary v newspaper article v story).
You could look at searching for "writing prompts" on Tumblr or Pinterest to have a bank of resources for practising at home, though.