We have one of those UV light things you see in kitchens etc out in the phils and they do seem to work. As Graham says screens over windows as well as screen doors is a big help out there as well. Obviously you need to cover food etc or at least keep it in the fridge out there as well otherwise you'd have an infestation of alsorts of creppy crawlies!!
I think we get flies in the UK as the nieghbour has two dogs which are kept outside so when we have our conservatory door open they come in from his garden.
