Lay out several sheets of foil to spoon the hot peanut brittle onto.
In a large frying or saute pan, mix the sugar, syrup and water together. Cook at a medium-high heat, stirring regularly. Heat until the mixture has a hard ball consistency on your sugar thermometer (121 to 129 degrees C), or until a small amount of syrup dropped into cold water forms a rigid ball. If you don't own a sugar thermometer, occasionally hold spoon above mixture, allowing it to drip off spoon. When mixture is at hard ball stage, it will 'hair' off the spoon when falling, looking like a clear spider web.
Once mixture is at hard ball consistency, add peanuts (they will pop a little). When mixture starts to turn a light clear-brown, as if it's starting to burn, remove from heat. Add bicarb and stir. Mixture will foam.
Spoon out bite-size pieces onto ungreased foil, stirring the pan constantly between each couple of pieces to prevent setting. Mixture can also be poured onto unbuttered foil as one big piece and broken into smaller pieces once cooled.