Inspired by @redgierob and his Literacy Shed Story Starters, and @Learningspy's Slow Writing task (http://learningspy.co.uk/2012/05/12/how-to-improve-writing/), the lesson went thus:
Once they had written their first, they moved onto a second sheet: here, the instruction was to begin with an adverb. On their next sheet, three simple sentences all under four words long. The next sentence had to contain some character point or detail, and be a compound sentence; and so on. I gave them 90 seconds thinking time at each sheet before asking them to commit to paper, which felt like a really long time, but they could have actually taken longer, as the task became more complex the longer the stories became.
After a lot of pondering, some clarification over sentence types and some discussion over clues in the text, the pupils then chose one final story for themselves. This sheet was the one they evaluated, wrote feedback on and discussed as groups and then as a class.
The lesson finished with them then taking the principles they had learned this lesson and writing the opening for their story.
All in all, it was an interesting lesson which threw up some surprising discussions about genre, immersing the reader and when writing needs to be tight or loose. Hopefully the pupils' work will improve as a result!