I have used these to great effect. They do in theory attract the female flies and this in turn effects next years fly population. Do not hang to close to the house as they do pen and ink. You can get them at garden centres and cooperatives but these on e bay even with postage seem cheaper.

http://www.ebay.co.uk/itm/1-X-RED-TOP-FLY-TRAP-CATCHER-ASHMOAT-SINCE-2004-/170824903051?pt=UK_Home_Garden_Garden_Plants_Weed_Pest_Control_CV&hash=item27c5f5298b

http://www.selfcontrolfreak.com/slaan.html