Thought I'd list some here to save you the bother of looking them up!  Feel free to add any codes.  Doesn't have to be reef related!

APC PURE (balling light chemicals for alkalinity etc)  -  20% off with code BLKFRI
Fish Street (chinese reef equipment) - 11% off with code T3BF22YK   
Charterhouse (marine equipment, food etc) - 10% off with code BF10
Swell (marine & freshwater stuff) - sitewide up to 70% off without a code
H2O aquatics (marine stuff) - 5% off over £100 with code BF5 10% off over £400 with code BF10
AAC Aquatics - 50% off KH Guardian   10% off Nero 5 wavemaker