examiners are lenient enough to look for your work if you write an essay in the wrong booklet (i've heard this from not the chief examiner, but a high up there one). if they will make this effort when you make a mistake big enough as writing in the wrong book, im sure they'll be lenient enough to make the effort to turn the page. especially if at the top of your page you wrote 'Plan'
thats what i did anyway.
also, theyre told to not look at statement of intentions which would be on the lined paper (even though thats context and i think we're talking text here), but point is, they don't just mindlessly mark anything on the lined bit. they mark your essay, because of the 40000 17/18 year olds, there will be small errors everywhere that they can't let get in the way of their marking. plan on first page included.