I usually give these a haircut in spring, after the catkins have finished, thinning out excessive old wood and trimming the bottom of the branches so they don't brush on the ground and generally reshaping. Don't go too mad though, heavy pruning results in excessive growth - and never cut beyond the graft point at the top of the straight stem, where all the shoots come from.

All we do is remove excessive undergrowth and prune growing branches that are attempting to walk across the road - yes literally!