Mate if you want a great dnb night, give Fabric a wide berth! It ain't the best on offer, in fact its pretty shit for the most part.
I thoroughly recommend this, on this Saturday

I am planning to attend)
Renegade in Brixton with the Valve System
breaks - there is often great gigs at Clockwork in Angel. Same goes for dnb too - it's a bit of a hotbed of wicked cheap proper nights.