Thanks everyone. I guess "comfort drink" is a good way to describe it. When I'm drinking it, my mind is tricked into thinking I'm drinking something more appealing by all that fizz!
I've had a brief look on wikipedia about the health effects of carbonated water. Apparently the only thing to worry about is the erosion of tooth enamel - but this is said to be "neglible".
I can't see myself carrying on with this forever - it's really just a cost-saving experiment for this month. However, if it's a huge success (both to our health and our wallet) I can see us doing this more often!
Cedars