i brought a small orange tree and it seems to be swarming with black flies don't know how to get rid of them

Are you sure they are flies Shaakh? The citrus blackfly is not endemic to the UK so I doubt it is them. May just be your common or garden (pun intended) aphid or perhaps it's red spider mite which isn't really red at all. If you are getting cobwebs then come back for advice but assuming it is just aphids then treat it with any greenfly/blackfly killer including bug clear again, or spray with a weak solution of washing up liquid a few times, a week apart each time, or physically squish them with your fingers or if you want to be really purist you can use a biological control which is probably too much for one small orange. Aphids are annoying and will weaken your plant, red spider can do a lot of damage very quickly so I suggest you act at once. I assume that this tree is being kept very warm? Can you describe it's habitat?

John

it is indoors at the moment as it can be kept in coservatory and can be kept outdoors i have washed it well in sink jsut with water as it was really bugging me most of them either drowned or flew off i have applied my normal feed to them today so i will have to wait a while before applying the bug clear again

HI, Check for Scarid fly, it sounds like this to me,
usually a sign of over watering,
the old compost is probly sour now
so suggest changing compost,