I'm a bit mystified by the lengthy discussion above, bemoaning how various DIY shops are good but not really in Brixton, and how Brixton is all full of eateries and no DIY shops, when the one that is right in the middle of Brixton has already been mentioned; it's on Brixton Station Rd and is perfectly good for most basic stuff.
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Brixton is not a DIY supplies desert. The appearance of Screwfix and Toolstation relatively nearby have improved the overall situation considerably, as the nearest ones were previously much further away, although I do wonder if they take business away from the aforementioned smaller shops.