Hi there Andy
It's a very good question and one I've been trying to find the answer to for some time as it's the one flavour missing from the dishes I make I just can't seem to be able to achieve and for me constitutes that missing 5%.
I know exactly the flavour you're referring to, but it's hard to describe.
There was an interesting discussion on this very topic I raised here -
http://www.curry-recipes.co.uk/curry/index.php?topic=6318.msg65981#msg65981Unfortunately it's long and convoluted and with plenty of fluff, but it's worth reading if you're determined enough to try and replicate this on a repeatable basis.
Unfortunately, despite what others might try and tell you to the contrary, currently there is no easy or conclusive definable answer as to how to achieve this particular flavour, assuming of course we're both referring to the same one.
I've recently run a few tests on dishes I've made, proving or disproving certain theories I have on what's producing it, and I believe I'm getting closer to achieving what I've been looking for.
However it's not reliable or conclusive enough for me to be able to give any details on what might produce it, given how savage the dogs here can be on anyone who dares to post an honest opinion that might possibly conflict with their own.
What I will say is, I believe this particular flavour is achievable in a home domestic kitchen when using the right ingredients and cooking techniques.
I wish you well in your search for achieving it.