How many blankets? One grobag is the answer!
Seriously, they're fab! We've used them ever since moving ds into big cot (from about 7 weeks). We had bought a whole load of blankets which have hardly been used - what a waste of money - but the grobags are worth it, ds would never stay under his bedding otherwise. I can't even get him under normal bedding in the first place - he just kicks and wriggles so much. First time he slept through was the second night he was in a grobag, but maybe it was just coincidence.