From Dan Toombs' "The Curry Guy" book:
TAKEAWAY-STYLE PAKORA SAUCE
MAKES ABOUT 300ML (1¼ CUPS)
If it’s Indian takeaway-style pakora sauce you like, this is a good one. Usually this sweet-and-sour sauce is red from food colouring, and does look the part when bright red. The mango chutney and ketchup are already quite sweet but you might want to add a little sugar. The sour flavours come from the lemon and mint sauce. Just try it and adjust until you are happy with the flavour.
1 onion, finely chopped
2 tbsp smooth mango chutney
3 tbsp tomato ketchup
1 tsp mint sauce
200g (scant 1 cup) plain yoghurt
½ tsp roasted cumin seeds Spice Blends
½ tsp chilli powder, or to taste
1 tbsp sugar
Lemon juice, to taste
½ tsp red food colouring powder (optional)
Milk (optional)
Salt
Whisk all the ingredients together, except for the food colouring, milk and salt. Once the sauce is nicely combined, check for seasoning and add salt to taste. Add food colouring, if using. If the sauce is too thick, stir in a little milk until you are happy with the consistency.
I think this is probably close to what you've already tried so maybe not much use, and you'd need to leave the yoghurt and milk out. I imagine you could add a dollop of this to a standard curry and make it a chasni as well.