The short answer
Most London hedges want two cuts a year: one in late May or June once the first flush of growth has slowed, and one in late August or September to leave the hedge tidy through winter. Fast growers like privet and leylandii often need a third cut in July. Formal box and yew are best clipped once in June and once in early autumn.
The nesting-bird rule
Wild birds, their nests and eggs are protected by law while a nest is in use, and in London the main nesting season runs from March to August. That does not make hedge cutting illegal in those months, but it does mean we check a hedge before we start and leave any section with an active nest until the young have flown. If your hedge is full of sparrows in May, the kindest and simplest answer is to book the cut for late summer.
Hedge by hedge
Privet: cut in May, July and September; it takes hard cutting well. Laurel: two cuts, ideally with secateurs or a sharp trimmer so the big leaves are not shredded; it can be reduced hard in spring. Box: clip in June and September, and feed in spring to fend off box blight and caterpillar. Yew: one or two cuts from June, and it will regrow from old wood if you need to reduce it. Beech and hornbeam: one cut in August keeps the brown winter leaves on. Leylandii: two or three light cuts a year, never into bare brown wood, because it will not regrow from it.
Reducing a hedge that has got too big
Most hedges can be brought down in height or width, but it is better done in stages over two seasons than in one go. Privet, laurel, yew and box all recover from hard reductions; leylandii should only be taken back to the green growth. We cut the top first to a line, then square the sides, and we always clear and bag the cuttings.
What it costs
Our hedge cutting is £60 per hour for the two-man team plus £10 per large bulk bag of cuttings removed. A typical front-garden hedge is well under an hour; a long boundary hedge outside a block of flats is usually a half-day job.