Ive finally managed to replicate the taste and smell I associate with BIR curries, apologies for the long rambling nature of this post in advance.
The catalyst for my discovery was Pete's recent post on the Madras and using the KD base. Below I detail all the steps I went through to get to my discovery, I have since cooked the curry again from scratch including base and it was exactly the same.
First off Pete got me all excited about spiced oil again and his talk of the KD base so I cooked the KD base with about 500ml of corn oil and skimmed most of it off to get my first batch of spiced oil. I cooked some good curries and not so good curries with the KD base and ended up throwing most of it away (Pete's madras being the notable exception), this left me with some spiced oil left over.
I got to thinking what do the BIR's do with left over spiced oil, of course they reuse it. What follows is my description of 2 weekends just gone.
First off I started on my next base with the spiced oil I had left over, I chose Dave Smiths base (premium area of curryhouse.co.uk) and his new recipe for Chicken Achar.
I added all my left over spiced oil and some new ground nut oil to start the base off, when I added the onions they were all but submerged in the oil, this had the effect of pretty much deep frying the onions in the base. At the end of the base I skimmed off the oil.
I cooked Chicken Achar using the reclaimed spiced oil and muttar paneer, both of which I reclaimed oil back into the spiced oil pot. These curries were superb and had the taste and smell, I decided not to post as I wanted to make sure I could replicate this.
Next weekend I started the base again (Dave Smiths base only makes 600ml) I started with the left over spiced oil again only this time I also added the last of the curry base from the previous weekend. My 'son in law' works at the bar of my local BIR and I asked him has he ever seen them pour in the remnants of the previous curry base when starting to use a new batch, his reply: always. I skimmed the oil off again.
Cooked the chicken achar again and this time even better. In summary I conclude the smell/taste is due to reclaimed oil not only in cooking the curries but also in the base recycled ad infinitum. Also recycled base ad infinitum added to a new batch.
I would recommend the premium area of curryhouse.co.uk to anyone, I will post the recipes above when I get chance but for those that already have access you need to look at the base, achar and the pre cooked chicken documents