When we first got our hens we were told that we needed an electric fence due to the amount of foxes in the area. We live in the countryside but I know in the UK they live in large numbers in urban areas. We have one that plugs into the main and now also incorporates my childrens rabbit and guinea pig runs. Two years later they are still ok and they put themselves to bed and get up as and when they choose.
I had a quick look on ebay and they are buy now for £60, the battery ones are much cheaper; about £30.
Just a thought - Allie