in northern minnesota we wait till night when there all sleeping (hopefully) and take a blow torch and burn them out.! gets them all!
A friend of mine swears that he has no problem getting rid of wasps by spraying them with a mixture of Dish washing detergent like "Dawn" and water. Now I researched this on the internet and there are quite a few people that say it works for them. One says the mixture to be a quart of water and 3 to 4 tablespoon of dish wash detergent. Hmmmm
That's an interesting solution and has to be cheaper than the cost of calling out an exterminator (not a Dalek). A few years back, we had a huge wasp nest in our loft...................nasty things, although I must say that their nests are quite remarkable.
My only caveat to Annie's suggestion is to try to set the dish/saucer so as not to attract honey bees, which are in decline in the UK.
Thanks Annie.