Bengali Bob,
i'll read up on the your post this week.
it's going to take some blinkers off thinking to crack this for me.
i am pretty sure there are no special ingredients or cooking utensils or technique.
the difference that always sticks in my mind in comparing to BIR is that the finished sauce is much darker and the spicing completely different.
consequently i think the difference is down to:
1) the base - uses more whole spicing
2) mix powder - this must be more like a chef garam
i've added below the info i've collated to date. i also have some info from unclefrank which i left out so he can input too.
the only thing i've tried is hot frying black cumin and black mustard seed in the oil. the mustard seed is no no but cumin could play a part.
Lynette Baxter Book
1) essential flavours onion tomato cumin coriander
2) dried fenugreek leaves add authenticity (? add to all, ?fresh)
3) sizzle cumin & mustard seed in oil at start of dish few seconds
4) lentils, Pulses, chick peas used in veg dishes
5) fresh Coriander special part stirred in at end
6) Crushed Garlic fried with chopped onion at start (crush needed to release full flavour chopping is quite mild)
7) garam includes mint, used in most dishes
8 ) no tomato puree most dishes
9) regular ingredients: green chillies, Ginger, green & red pepper, lemon juice
10) mix powder or garam or both
BBC - Lenny Henri Video (from Natterjak)
Sauce, methi, garlic and flour, fresh tomato, cumin, coriander
Birmingham balti co
In short comes down to the "balti masala" mix powder
Back cover) - 5 star recipes to cook at home
12) Herbs - fresh (coriander, mint, pg 94 methi). Don't use methi stalks bitter.
19) balti masala "mix powder" has extensive ingredients: whole; Coriander, cumin, fennel, black mustard, fenugreek seed, clove, bay, curry leaf, lovage seed, onion seed, cassia (no cinnamon) methi. Ground; chilli, Ginger, cardamom, garlic, turmeric
20) Chef garam - aniseed and cardamom listed as ground not whole spice and not roasted. Ingredients: whole; coriander, cumin, cassia, mint leaf, bay. Ground; aniseed, cardamom, clove, turmeric, paprika. Would expect turmeric & paprika better in mix powder
62) cassia bark piece used at dish frying
66) black cumin seed, onion seed used whole at dish frying
67) garlic:Ginger 50:50 in dishes. Ginger not always added
96) 50:50 mix pwdr:tandoori masala in madras