Evil little sods! They had a nest next door to my MIL's place & swarmed into the kitchen one hot summer's night when I was washing-up. I legged it, shut all the doors & called the council the next morning.
The pest-monkey turned up, did something up the ladder then ran like stink when they went for him. He returned in protective gear & got rid of the b*ggers - cost, about £30 as I recall.