I'm trying to grow a lavender hedge too, I had a go a few years ago when I first started gardening..I, in my inexperience, cut the lavender back really hard but far too late in the year, not one plant survived. As it's a fair few plants needed I just couldn't afford to buy a load of them... There was a deal on for 25 plugs for next to nothing from an ad on gardeners world so I've been raising them and will have another shot at it.
So I've learned not to prune to hard to late. I hope this is useful to anyone thinking on it..pretty obvious, if you know I suppose.
Another lavender horror story I came across was of a Chinese lady, a gardener and a garden centre. The lady wanted 50 lavender plants right round her garden, a lawn and all those lavenders and nothing else. That's fine...whatever she wanted was her choice....actually sounds quite nice...if you had acres of land you could have a dedicated lavender garden, anyway the gardener I was working for asked for advice from the garden centre on how to plant it, they said good drainage and good compost, so he did all that.......all 50 plants died!! I believe he planted another 50 plants and they all died too....I've seen this chap I do occasional work for but daren't ask how that saga turned out, a very sore point indeed.
So I learned not to fuss lavender, I hope he did too. I just plant mine in any old soil after that, the less fuss the better in my view.
Sorry it's a bit long winded but hard lessons to learn and best shared.