Peter, I think we are of the same mind here just looking at the subject from different sides of the room.

Almost disturbingly coincidental, I have taken a phone call this morning from my local, here at work (we supply them certain things). This then turned into a trip to the restaurant.
During the course of my visit, I was able to ask questions to both the owner and his cousin, get close to the stock and have a look at what they put into it.
During my conversation several things became clear. Firstly, they have a passion about their cooking and were very happy to talk food. But it was clear that the language barrier was going to be a problem as much as they admitted, they didn't know the English Terms for many of the ingredients.
By far though, the greatest passion they had was trying to educate me that cooking BIR is not the way to cook curry. Time and again, this was mentioned and time and again I was offered to see how a 'real' curry is made.
Perhaps both of these and the odd pause to talk in Bangladeshi with each other, was why I felt they were being cautious of my reasons to ask. But I also felt that they honestly did not know all of the answers. Perhaps these are the simple reasons that I feel everything is shrouded in mystery.
ObesrvationsSpice MixOn entering the kitchen I made several observations firstly, they had more than one spice mix. two larger containers held two very different mixes one with an orange/red appearance with traces of dark spice within it and the other a much lighter yellow/orange colour. There was also salt, sugar, what looked to be coconut powder and and 1 or 2 others.
StockThis is where I now have to admit that I have been wrong to call it stock. Infact, my original observations (from some years ago) must have been wrong entirely. What was indeed in front of me was a pale yellow sauce which clearly wasn't translucent. On top lay a film of what I can only describe as oil and skin. Its consistency was that of say a Heinz tomato soup.
Out of ViewI didn't think to look under the main prep table but during our discussions, I was shown from underneath an ice cream tub full of what was explained to be tomato puree. Clearly it was not just puree. It had a very smooth texture and a rich red colour. Traces of deep red oil were trying to gather at the edges of the tub and I could see that something was submerged within it, probably garlic cloves from what I could tell.
DiscussionsThis really is in two parts because the spice mix deserves its own mention. This is as it was explained but does not provide the full picture i'm afraid. Fortunately though, I have been invited to see the gravy made from start to finish, which I intend to do.
Gravy MainThe bulk as we know is onion roughly chopped the process was described: Using about 40lbof onion (this seems like alot) add to pan with a little water and soften, then add a mixture of ginger, garlic and green chilli that has been crushed, about 2 table spoons and again along with green peppers and soften. Top up with water and cook for 2 1/2 hours. Blend the mix thoroughly and then add the spice mix. It gets quite vague here as he wanted to talk 'real' curry and I wanted to press on with the procedure. Sadly that's as far as we got.
Gravy Spice MixHe lead me to 'the shelf' where bestowed upon it rested an assortment of spices. Some usual suspects and some not so. I was told that all of these were used to create the gravy mix. Among these were Cinnamon, Coriander, Bay Leaf, after that it got a little more involved. Firstly he mentioned Jeera regular seeds as you would expect but also Black Jeera. Then there was what he called Aniseed which is not star anise but a seed similar to fennel but smaller they call something like 'guam moori'. Then a mix of spices they referred to as Punch Puron. This actually contained a mix of spices essentially this is where the normal Jeera was but also with black and yellow mustard seeds as well as onion seeds and possibly others.
ConclusionI was wrong about the stock, which is a relief in a way as it obviously challenges almost everything I have read on here. There are quite a few spices involved in the gravy alone and more than one spice mix is used, as well as at least one other shortcut procedure, the tomato puree mix.
I will report back with more when I finally get the chance to watch the sauce made from start to finish.
Thanks for reading.