Soldering Mains Cables and Joining

[gubbabump]

Hi everyone - possibly a daft question but would rather make sure.

Ive got a couple of Xmas and halloween decorations that I want to put up this year (I know its too early!!)but whilst im cleaning my garage out I thought Id do the task I put off every season, which is actually extending the cables so they reach a socket.

I have experience in soldering audio cables and little leds up etc whilst at college, but ive never soldered a mains cable plug.

firstly is it safe to do so? the original cable is 1.00m2 and has a diameter of roughly 6.5mm and I have purchased the same wire to connect it to, I have also have shrink tubing.

My question is - what kind of 'solder cable' do I need? ie what diameter and thickness (I presume its just the same diameter as the cable?) and is there anything else I should know.

thanks everyone.

[gubbabump]

I perhaps should've mentioned the 'join' would be indoors and not outdoors. un less im looking in the wrong places lol there isn't much online regarding joining mains cables together other than using 'choc blocks etc' I just presumed soldering was better.