1. Soak tamarind in boiling water for 20 minutes.
2. Push through a sieve or mouli. Discard pulp.
3. Add rest of ingredients to tamarind liquid and put in a large heavy-bottomed pan.
4. Bring to the boil and boil for 20 minutes or so until thick (ish) but not too thick. It will be shiny.
5. Bottle in sterilised jars. Store in fridge once opened.