Claire~
I have had this exact problem before...
The ants come due to the contents being a little on the dry side and also being too acidic.
The solution; Keep the worm bin a little more moist and secondly, buy yourself some cheap builders lime or dolomite lime from the hardware and give it an even little sprinkle over the feeding surface of your worm bin. This will bring up the pH a little, help remove any smells and make the worms happier and hungrier. Pretty sure you'll need to get another load of worms though because five aint' gonna eat much
I was informed by a great neighbour of mine who used to work part time as a kid with a commercial worm farm, this does work.
Hope it helps,
Ben