First thing to do, you need to check that your ‘swarm’ are indeed honey bees. A wasp’s nest, for example, will need dealing with through your local council’s environmental health or pest control department.
The swarm itself will probably be about the size of a full-size rugby ball. The swarm will hang in a tree or a bush or under the eaves until they have found the kind of home they want. This could be your chimney stack so don’t delay.
Take a look at the advice on our sister organisation, BBKA’s website, this might help identify what you see. http://www.bbka.org.uk/help/do_you_have_a_swarm.php Thanks to them for the information.
If they are a swarm of honeybees then below is a link to a list of swarm collectors in your area. Hopefully they can help.