Darth, I suspect it's the big spanish onions as I can't see the BIRs messing around with the small ones - hopefuly Mark can confirm. I used 10 of these (about the size of an orange) along with 2 big carrots, 2 sticks of celery and 1 Green pepper for this amount. I also added lots of boiling water - covered the veggies and then an additional inch. The consistency of the blended base is spot on - not too thick.
Pete, I haven't actually tried a celery/carrot version yet but this is by far the best base I've made and I've made hundreds in my time. I think it has great potential to have the taste, but I need to wait until I try the finished curry to be sure.
Joe, As you say, this is only my opinion so everyone needs to try for themselves. However I did remove my BIR water wings many years ago and have been reared on Glasgow's finest curries since

PS. Not sure if this is really significant, but I also added a couple of tablespoons of Knorr 'Touch of Taste' liquid chicken stock concentrate. I find this added some depth to my previous bases.
Oh, and my kitcken smells like a BIR. I can't help smiling every time I walk past the pot
