I did use a red onion but I've never found that the type of onion has any significance in a curry. I would be just as happy to use a brown onion.
I used the first of my home grown Tiny Tom tomatoes (about 10) and I included about 1/2 cup of water in the blending process so I did already have some extra liquid. The resultant puree seemed a bit pale so I added a heaped tsp of concentrate tomato paste. I would confidently replace the blended whole fresh tomato with a tinned product, passata, puree or diluted concentrate paste. I don't think it would greatly change much.
Everything else you transcribed is the same as I did Phil. I agree that it is not BIR but it is a really good traditional Chicken curry.