I always use either tinned tom juice or the 'blitzed' contents of tinned toms ( a tin of plum tomartos blended with their own juice).
Firstly I fry a chopped chilie "Bruce Edwards'" style, add a small ladle of base to help prevent the spices burning, add the spices and fry this right down, and repeat with another ladle of base. The process seems to caramelize the sugars in the the pureed onion within the base, and consequently adds to the flavour; as well as cooking the spices to 'toffee' without burning. The the chopped chicken goes in, I don't pre-cook chicken 'cos it only takes 10 minutes to cook.
Next I add the rest of the base in stages, allowing each addition to come up to the boil, TOGETHER with the tinned tom juice a a quarter tps of worchester sauce (no more than that, that's important!), total base volume is 450ml at start.
I tried pretty much everything in my currys to get the right sourness: vinegar (wine, rice, malt, cider), lemon juice, lime juice, tamarind paste. Nothing worked really well until I tried tomarto juice. I got the idea from Camella Punjabies book (traditiona Indian stuff), in which she stated that tomarto can be used for souring as well as sweetening. I use tom puree for sweetening (bhunas stc).
I was was quite pleased to note that C.K., who writes a lot of good sense stuff here, had found the same thing, although his technique is slightly different.
I am REALLY happy with my madras now, have have moved on to trying to get other dishes right!
I wrote the method I use in the C.K.s madras thread somewhere asI recall, I hope he didn't mind, I wasn't trying to steal his thunder so much as reinforce what he said. But in any one is interested my spice mix for madras is:-
-4 lev tps Bruce Edwards' spice mix (with paprika), using Rajah mild madras as the curry powder (the hot madras is not necessary as chillie goes in as well, and anyway it tastes to strongly of bayleaf.
-2lev tps Rajah extra hot chilie powder
-1/2 lev tps (or just less than that) salt
-1 lev tps (or a tad less) brown sugar
-1/4 lev tps of GROUND dried methi leaf.