Lighting circuit problems

Hi all, I've been lurking for some time now, and have found loads of useful info, but now I have a problem!!
At the weekend I cladded the kitchen ceiling, which has one ceiling rose at each end. Saturday I removed the rose, cladded and refixed - everything working no problem. Sunday I completed the job, removeing both ceiling rose and pull switch along the way, and replacing EXACTLY as it was.
Well now half of the downstairs lights refuse to work, although a voltage beep tester thing indicates there is some voltage up in the roses of the non working ones. The fuse is fine- all sownstairs lights are on the same one, and surely if I had inadvertantly nailed into a cable, I would have known about it?
The house still has its original wiring, unfortunately - the pull switch, which seems to be some kind of master (because similar houses to mine have had all sorts of weird problems when wired incorrectly), has three wires to one terminal, and one to the other. Unfortunately all red!!!!!. Now obviously this worked originally, so I am boggled!
A spark friend of mine cannot understand the problem over the phone, and I can't afford to get one in, so can anybody shed any light on the subject? I am normally quite competent with electrics, but cannot get my head around this!!
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You've probably chopped a cable, or it has slipped out of its terminal. The beep voltage tester is about as useful as a chocolate sock. Open up the boxes and make sure all the cables are tightly held by their terminals. If the kitchen light works, it is probably the circuit loop out that has failed. If it doesn't, it is probably the loop in.
Christian.
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Cheers, I'll check taht over the weekend. Only just been back to check for replies, which is surprising as the wife mentions the lack of lights every two minutes, whilst also insisting that I install cladding and spotlights in the bathroom/toilet at the same time(which was a perfectly successful project, i may add!!)
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