A strong swarm may contain up to 25,000 bees and will appear as a large buzzing, swirling mass, until they settle when all will go quiet. Bees normally swarm at about mid-day on warm/hot sunny days and will appear from then until early evening.
If a swarm appears in your garden, keep children and animals (dogs and cats) indoors. Please make no attempt to frighten the bees away.
Do not spray them with insect killer, a hose pipe or pour petrol/paraffin on them. See if you can see where they have clustered (and, indeed, can it be reached by ladder etc?). Then call one of the following local beekeeper representatives, who will either come themselves or endeavour to contact another beekeeper in the close vicinity to collect the swarm. When you call, the beekeeper will need to know the address and possibly the post code of its location, your name and phone number, and how accessible the swarm is. If the swarm is not reasonably easy to reach, a beekeeper is unlikely to be able to help you, but the bees will probably soon fly away. If accessible, swarms can be collected and transferred to a hive. If successful, the beekeeper will leave the bees in the hive until later in the evening, when all the remaining flying bees have found their new home and then take them away to a new location.
If the swarm does fly off after you have called the beekeeper, please do call back and tell them, to save an unnecessary journey.