I have just completed a 20 foot length of standard yew hedging. The plants were a good buy of 5 per pack from our garden centre and are really sturdy plants around 30 inches high.
I intend to keep it tightly clipped and narrow once established and around 5 foot tall. Do I take out the leaders now and clip the side stems back to thicken it up or wait until the plants are a lot bigger?
Don't take out the growing tip of the leaders until the hedge has reached the height you want.
Once your new hedge has developed a good root system (which will take the first growing season) it will grow quite quickly at up to 30 cm per year, but once the leaders have been taken out the rate of growth slows from about 30 cm per year to about 5cm, so you must let it reach it's final height before doing that.