This is what the RHS says. but enjoy next springs display then prune.

Witch hazel don’t need regular pruning – it’s best to let them grow freely. Simply remove any dead, damaged, congested, crossing or weak shoots.

Restricting size

If you need to restrict the size of your witch hazel, the following steps will help.
■After flowering, cut back the previous season’s growth to two leaf buds from the main stem
■Leaf buds are longer and narrower than the more rounded flower buds, so try not to remove the flower buds in the process
■This pruning will encourage new extension growth and also promote flower bud formation at the base of the new shoots

Gardeners World last week, Toby Buckland was showing how to encourage growth further down if the stems are bare at the bottom....Run your eyes and fingers down bare stem, find the little knobbly joints , preferably outward facing, just below said joint, using a knife cut into the bark making two slits , only a couple of centimetres apart one below the other, then peel off the bark exposing the green stem, apparently this will encourage new shoots to burst forth....

With that technique you trim the bark off just above the knobby joint. That stops the flow of hormones from the growing tips, so the plant thinks that that branch has been eaten off by an animal, without stressing the plant as much as an actual pruning. Over the next year or two, the treated branch will gradually die, and can be removed. I would do that to about 1/4 to 1/3 of the longest branches, and repeat every year or two, until all of the older branches have been removed.
Sorry Lincslass, but Mr. Buckland may have made an error: the way plants grow, removing the bark below the joint would simply mean killing that joint, leaving a dead stub down to the next joint down--risky for diseases.

Tug Toby did explain about tricking the hormones, therefore encouraging the new growth, I apologise, I've reread and I had missed that bit out, (Goy playing up and won't let me edit today )yes cutting below is to stop the branch sprouting, above is to encourage the growth. I know it works on sme as I did it on my Sambucus, does not work on Buddhlia, I tried to stop it blocking my path but then nothing seem to stop them, lol....