Fitting a Bathroom lighted mirror

[inneedofhelp]

I want to fit a lighted mirror in my bathroom, but the existing wire running to a wall light is too short. Is there a way to extend this existing wire safely or do I need to replace the whole length by tracing it back to a ceiling rose ? Thanks

[inneedofhelp]

It has been suggested that I could install a junction box between the ceiling rose & the new mirror. This would mean I wouldn't need to touch the conections in the ceiling rose. I would simply cut the existing wire install a junction box in the loft & connect the existing wire to it, then run a new wire of the correct length to the mirror from the junction box. Is this OK ? Do I simply connect black to black, red to red & Earth to earth ? Is it a standard 20A junction box that is required ?

Thanks

[THRIPSTER]

Upon reflection, I think you should call in an electrician.