Place water, sugar and tamarind pulp into the Thermomix bowl and cook 8 min/80°C/speed 2. Cool to just above room temperature.
Add lemon juice and stir to combine. Taste. See if you think it needs more lemon, tamarind or sugar. It will continue to grow less sweet as it ferments over the coming 48 hours. Cool to just slightly warm.
Add ginger bug and stir, pour into sterilised glass bottle with a sealed lid. Allow to sit at room temperature for up to 3 days. Check for activity by shaking it occasionally, being careful not to over shake it once it starts to produce gas. You should have a reasonable amount of room at the top of the bottle. Burp the lid if you think it needs it.
At around 48-72 hours it should be fizzy enough, so transfer to the fridge for several hours and enjoy icy cold.
We absolutely loved this one. You can dilute it to taste with sparkling water. It is quite concentrated.