Why have I got volts (38v) When light is off

Where are you reading the 38 volts?

On the light fitting

Which light fitting, and to where.
If you have zero volts with the light switch removed, the switch is bad.

38 volts on the ceiling rose,when switched off, 240 volts when switched on.

Where are you located?

Tried a new switch, was not that.

If in the US, and you are getting 240 volts, you may have a loose neutral as well.
If you are measuring low voltage off the rose, that may be a ground problem.
You should have 120 volts between the shell(threaded part), and the center.
Must be careful and aware it is dangerous if both probes touch when measuring.