Jerry
I use extra pureed tinned plum tomarto in my madras, someone else here does this also(CK perhaps?). It adds a extra very slightly sour 'tomarto-y' taste with the cloying sweetness of puree. About 3 tbs per 450ml of base. You need some puree too.
Another avenue that might be worth exploring is tomarto ketchup. Suprising I know but lots of British Chinese Restaurants (BCRs ??) do that. It add a differnt tomarto note again, obviously sweeter (than puree even), but it might be worth exploring. Madras from 'The Vogan' in Merstam (near me) tastes of ketchup. I wouldn't be at all surprised if BIRs used it, it's cheap and easily availble. After all some of them use cream of tomarto soup and Worchestshire sauce!