This is a recipe for Tamarind ("Imli") Sauce adapted from Diane Lowe's and Mike Davidson's "100 Best Balti Curries". 

It is allegedly a recipe from a balti restaurant called "Mokham's" in Birmingham.  Serve it with Poppadoms and Pakoras.

Ingredients:

• 200g block of compressed tamarind ("imli")
• 1 small onion (roughly chopped)
• 0.5 green capsicum pepper (seeded and roughly chopped)
• 1 tsp fresh coriander leaves (roughly chopped)
• 0.5 tsp salt
• 1 tbsp sugar (or jaggery)
• 1 tsp concentrated mint sauce
• 1.5 tbsp tomato sauce (ketchup)
• A few drops of red food colouring (optional)

Method:

• Break up the tamarind block and put it into a bowl with 300ml of water
• Leave to soak overnight
• Squeeze the pulp, to extract the juices, and strain the liquid into a clean bowl
• Put the onion, green pepper and coriander into a blender and blend into a paste
• Add the rest of the ingredients and tamarind liquid and blend until smoothe

quick Q , what is tamarind

Tamarind ("Indian date") is like a date fruit.  It is highly acidic and is used as a souring agent in Indian cooking. 

It sours, thickens and darkens sauces (it can be typically used in a patia, with sugar, to produce a sweet and sour curry sauce).

You can buy it in a compressed block (from an Asian store).  You soak a chunk of it in warm water to dissolve it.  You can also buy it, as a concentrate or paste, in a jar.

http://en.wikipedia.org/wiki/Tamarind
http://www.hort.purdue.edu/newcrop/morton/tamarind.html

made this tonight as part of 3 sauces/dips  was very nice thank you