Many years ago whilst working as a dishwasher in a North Scottish BIR, i wish i had got the whole recipe, they made a sauce which i assume is similar to what you are all after, this sauce was served with all starters and is quite common up here.
One of the main ingredients was tamarind water, other ingredients ranged from tomato sauce/chopped tomatoes with some tomato puree, mint sauce, onions, coriander, sometimes they would throw in bell peppers too and blend the lot.