At least as many research studies show a positive effect of grammar instruction (see www.phon.ucl.ac.ukhomedickwriting.htm). What the research does show is that it all depends on how you do the grammar teachng. Decontextualised grammatical exercises are indeed a waste of time for most children, but they're not part of the National Literacy Strategy.
There is strong evidence that carefully targeted grammatical instruction, as in the NLS, can be extremely effective in improving writing.
Phonetics and linguistics department
University College London