I made a batch of this base a few weeks ago.  I've used it twice and it's been excellent - in a Saag and a Jalfrezi.

It smelled great while it was simmering and was very reminiscent of an east Asian sweet sauce.  A certain sweetness is something that I have noticed in most BIR curries so I was excited at the prospect of using it (prior to this I'd used some Pat Chapman recipe years ago which was bitter and awful).  I noticed that the smell changed a bit when I added the spices (some of the enticing sweetness was lost) and it made me question whether we should really be adding spices to the base sauce?.  I mean they're only simmering for 5 mins and I wonder if they're really contributing much to the depth of flavor at this late stage of the cooking?  I was always under the impression that spices should be fried in oil rather than boiled?  This is not meant to be a criticism of this recipe at all but I'm wondering if spice is really necessary in the base and if it might be better to control the spices at the main curry cooking step of the preparation?

Cheers