Since trying mango chutney in my pathias and dansaks i am convinced a lot of places use it, and not necessarily just in those two curries. Of the two best takeaways near me i know they both use it because i can taste it. The dishes with chutney in taste better and are the ones my friends agree 'are the real deal'. It's not necessarily a secret as i have seen it listed as an ingredient on menu items.
I use TRS sweet mango chutney which contains mango, sugar, salt and acetic acid as it is the type used by BIRs. ?4 for 3kg.