Potassium Metabisulfite can be added at several different times during the winemaking process, depending on your style/beliefs.
First, is in the very beginning, to stun any naturally occurring yeast or bacteria present on the fruit in question - usually at the rate of 1/4 tsp per 5-6 US gallons. This results in the winemaker having to wait a small period of time (12-24hrs, depending on the dosage - overages mean waiting longer) before being able to pitch the desired yeast. But, allows the winemaker to be more confident that the strain chosen is the dominant one, expressing the desired traits.
If working with fresh grapes (something i havent done yet, keep in mind), i believe its common practice to mix the sulfites into the must just prior to pressing the must off of the fruit. This prevents oxidation during the exposure to the air.
Sulfites are also used as a preservative for aging wine - after fermentation is completed. I dont know if your supplies overseas are the same as those allowed here in the States but Potassium Metabisulfite is usually added at a rate of 1/4 teaspoon per 5-6 gallons, every 3-4 months of aging. Some add less, or close to none - depends again on your philosophy in winemaking.
These are the more common uses for the additive, hope this helps with the information you were seeking.
