I eventually got round to making it ;D. First impressions of smell...Faaantastic. My oldest son , who doesn't know the hours I spend on this site, and unprompted, commented that it "smelt like a proper Indian restaurant"
I followed the recipe described almost to the letter! Except Morrisons sell packs of peppers cheaper than buying individually so mine had two green and one red. I also left out the chili powder. Everything else as stated by Darth.
For anyone else contemplating the 15 onion method you can JUST fit it into a 24ltr cook pot (If you have one) Saves splitting the oil, ingredients and spices between two pans which I think would defeat the object of volume cooking. (If that is the secret!)
When cooked and blended the consistency was that of tinned soup but after cooling, thickened to perfect

The colour however wasn't the Golden Yellow you described Darth How can I say... more like a babies nappy fill but hey the taste is great. We can't leave it alone. My son even had some with a slice of bread.
I am really excited at what this is going to make.
I have some images of the pre,during and blended sauce and have tried to post them but unfortunately can't. If anyone can tell me a step by step guide to resizing and posting I will put them in the pictures section.
Thanks for the inspiration Darth.