That isn't necessarily an indication of anything wrong/faulty. It depends
how the voltage detector works. Many such devices will 'detect voltage'
due to capacitive coupling between wires.
Chris Green- Hide quoted text -

I borrowed a digital multimeter. With the circuit breaker for the
lights off and with the socket circuit switched on, it displayed 28
volts ac. This fell away over a period of about 10 seconds down to
zero.

From what you said, I presume that it was induced capacitance in the
circuit.

Many thanks for your help - all of you.