Hebes generally respond well to hard pruning, they sprout new shoots from lower down the branches. I have cut mine back in spring, summer and autumn with no problems but frosty weather has been forecast so I don't advise you do it now.
I believe spring is the recommended time for pruning hebes. I'd take off a third to half of the shooting part of the stems then wait to see what shoots you get from further down. When the new shoots are good and strong then you could prune the top ones back a bit more. By this time next year you should have a nice shape to it.

We've found that Hebes don't mind being pruned really hard too. My OH got carried away and pruned a line of them down to stumps one year. Did them a power of good, in the long term (they were a bit of a "sight" for a while though). I don't recommend being quite that drastic with them, but if you lose your rag with them they'll come back.