I finally made it along to Siras in Southall today. Simply mesmerising range of spices, it took me a full hour to browse the shop! The fresh produce left me a little underwhelmed with the quality (or lack thereof) as their green and red peppers were not firm and almost starting to wrinkle and the fresh coriander bunches had black rotting dead leaves mixed in with them.
Next I went 2 mins down the road to Quality foods and to be honest preferred the place. It's an even larger shop, nice light and airy atmosphere and the sample of fresh goods I browsed were better quality than in Siras. Both places had an ample number of mainly asian shoppers though, so they must be doing something right.
I somehow failed to find Mr Naga in either shop, but did pick up some alternatives. Thanks for the info Bob, these are definately the right places to be shopping for curry supplies.