When I read the voltage at the bulb end of a light it is giving a reading of 6 volts AC when it is turned off and 240 volts AC when it is switched on. Is this correct as I assumed it would be 0 volts AC when turned off.
No, at the moment there are no bulbs fitted I am just reading the voltage on the cable where the lights will eventually be fitted. There is a 3 way switch and a dimmer switch fitted on the lighting circuit and all read 6 volts AC at the end where the lights will be fitted, but at the moment none are fitted, bar one.
So you are measuring the potential difference (voltage) between the 2 contacts where the bulb would be inserted? (and not between the line and earth potential).
Could be the impact of the dimmer when subject to an open circuit on its output.
Unlikely to be anything to worry about - you are only drawing a few microamps in making this measurement and voltages floating in this way when subject to virtually no current drawn do happen.