The place of evidence to inform educational effectiveness has received increasing attention internationally in the last two decades. An important contribution to evidence-informed policy has been greater attention to experimental trials including randomised controlled trials (RCTs). The aim of this paper is to examine the use of evidence, particularly the use of evidence from experimental trials, to inform national curriculum policy. To do this the teaching of grammar to help pupils' writing was selected as a case. Two well-regarded and influential experimental trials that had a significant effect on policy, and that focused on the effectiveness of grammar teaching to support pupils' writing, are examined in detail. In addition to the analysis of their methodology, the nature of the two trials is also considered in relation to other key studies in the field of grammar teaching for writing and a recently published robust RCT. The paper shows a significant and persistent mismatch between national curriculum policy in England and the robust evidence that is available with regard to the teaching of writing. It is concluded that there is a need for better evidence-informed decisions by policy makers to ensure a national curriculum specification for writing that is more likely to have positive impact on pupils.