Whenever working around electrical appliances such as ceiling roses, sockets and switches ensure that they are turned off at the consumer unit. As an extra precaution place a piece of masking tape over the switch on the consumer unit to stop it being turned on without consideration. Also write on the tape date, time and name of the person who turned off the mains and in what room they are working. This will help to prevent someone coming in, thinking the fuse has tripped and turning the power back on whilst you are working around a light fitting upstairs. If in doubt call an electrician.
A mains tester such as this one does not require contact with bare wires. Test it first on something that you know is live such as a kettle cable when the kettle is on.