There is a guide to extended written tasks in the Personal tax Performance Feedback 2015 (pages 17-19). Although it's written for Personal Tax, it has general recommendations as well, such as not to use abbreviations, etc. That's the only AAT guide to writing tasks that I know of.
You should also download and read the Performance Feedback for Financial Performance (if you have not already done so).
Answers
You should also download and read the Performance Feedback for Financial Performance (if you have not already done so).