I now believe that the fast reduction of the first ladle of base and subsequent scraping and stirring helps to create the smokey flavour and I find the toffee aroma is present at this stage.
As I posted elsewhere, I now mix up oil, base , G&G, spice mix, tom puree, chilli at the start of cooking and reduce the first lot of base so there is virtually no water showing.
At the same time some of the base is catching on my wok and going dry and brown - I believe that the onion in the base is caramalising when this happens and it has to be rapidly stirred back in.
I've definitely got more of a BIR flavour since adopting this technique.
I hate to admit it but until this point I was using teflon non-stick pans and now would not recommend this to anyone trying for BIR curries.