Get an electrician it's safer - or - buy a Mains Tester Screwdriver for less than £1. A neon light tells you if mains electricity is present when you touch the top. Undo the ceiling rose cover and among the jumble of wires three of them will be Red and all connected together. TAKE CARE. One is mains in from a previous rose - one is mains out to the next rose in the circuit (if only 2 red wires then it's the last in the chain) and the third Red is the supply to the switch. The live electricity comes back from the switch along a Black wire which should have a red sleeve on it to show that it's not what it appears to be. This black wire will connect to the flex Brown that goes to the lampholder. Flex Blue will connect to the other Black (neutral) wires (one from previous rose and one to next rose).
Use the screwdriver to diagnose the fault - if the neon lamp does not glow when you touch the top of it then there's no power getting to the rose - a wire may have become detached at the previous rose in the chain - check other roses. B&Q sell ready-made flex/lampholders quite cheaply.
Three Red wires - all connected together.
Three Black wires - two connected together and one (with a sleeve) connected to Flex Brown - Flex Blue connects to the two Black wires.
Get an Electrician it's safer.
Richard Buxton
September 2008