If it tastes sweet at all, there should be enough residual sugar to ferment to cause the fizz. I would suggest you make a starter batch of yeast - say 100ml warm water with a half teaspoonful of yeast and two teaspoonfuls of sugar. Once this is active and frothy, spoon about half a teaspoonful into each bottle (leave a couple of inches of air at the top) and leave bottles in a warm place for a few days. Try releasing the cap on one of them after say, 5 days and if you get an audible 'pssst' you're on to a winner. Now store them in a cool place, preferably a fridge if you've got room. Open carefully and pour into glass in one move, watching the sediment at the bottom and stop pouring before the sediment gets near the bottle mouth. It is not a disaster if some sediment gets into your glass - it just spoils the look and you have have to visit the loo sooner than expected....... Cheers, Andy