After some more reading, I realize what I didn't get the first time. CBM's is just a precooking method, but because it suggests a watered down tomato and then "add water until cover", it sounds more like a list of ingredients for both parboiling and cooking the bombay aloo.
today I tried to make bombay aloo starting from a minimalistic recipe from bbc's website:
Quote4 tbsp oil
? tsp mustard seeds
2 pinches of chilli powder
? tsp turmeric powder
350g/12oz potatoes, boiled and quartered
salt to taste
As I read someone in here that there is no noticeable difference between mustard seeds flavour, I chose to use yellow. I parboiled the potatoes using 1tsp of turmeric, 1 half onion, 2 garlic cloves 2 tbsp of seasoned oil. After frying it for a few mins to seal the potatoes, to add a crispiness around the edges, I was starting to feel a bit happier with the looks but when I tried the the potatoes I thought it was quite bland. So I added a little extra seasoned oil and 4 tomato halves. The looks were improving greatly, but the taste wasn't penetrating the spuds still

. After 2 mins, I added half-laddle of base sauce (I was trying to avoid the "potato curry" here) so then I let the potatoes finish to boil in the sauce and mix it often, to try and make that thicker and oily sauce. I also added a few more thinly cut onion and 3 watered down tbsp of tomato puree (w/o masala). Then I covered it and let it simmer. I can't say the potatoes were the best I had, but they tasted decently. At least, it sort of sets the mark to my next attempt.