Just bought a new GT mountain bike, and there is a few inches "free"

[fritz_le_cat]

movement before the crank catches.Normal? I've seen this in old, cheap bikes. I have to turn the crank 5-10% before it engages. And when peddalling with only slight pressure, it seems to "slip" when the cranks reach the vertical position. Not sure if it's the crank, or the rear sprocket that's the problem. I am pretty angry, but no point ranting in the shop if all the other model bikes have same problem. Any advice? (apart from never buying anything from Halfords - the McDonalds of bike shops...) It's mid range in price, with shimano gears.