If they're blackfly, some people will tell you to dilute washing up liquid in water and use it as a spray, but unless your washing up liquid is eco friendly, this is as bad as using a pesticide spray. You could try washing them off with the hose sprayer.

It sounds like blackfly, a kind of aphid. The best organic remedy is a thorough spraying with mild, soapy water, three or four times, two to three days apart. Sometimes they will go away with a vigorous spraying with plain water, but usually they're a little too tenacious for that.

As Bamboo says, if you go down the soapy water route you must use a vegetable based soap (like Ecover) or you can do all kinds of damage to the plant and the soil.

You can also try any one of a variety of "home brew" plant based sprays. There's some info about some of these in this article at Ecocharlie:

http://www.ecocharlie.co.uk/blog/index.php/2010/03/08/plant-pests-%E2%80%93-ten-top-organic-solutions

The garlic & chilli sprays definitely work very well - I've used them successfully on aphids, sawfly & caterpillars, and I like the idea of using the defences of one plant to help protect another.

Don't know if you're interested, Ilex, but I read yesterday there's a company with a treatment for bleeding canker in horse chestnut trees - the treatment is allicin, extracted from garlic because it's such a good anti bacterial anti fungal.

Thanks Bamboo, that does sound very interesting, particularly with the rapid spread of disease in Chestnuts. Plants almost always have the answers!

Yhank you everyone,will do.