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.

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.