Sonos does sell a longer power cable for the One:

https://www.sonos.com/en-us/shop/angled-power-cable

Hi , yes I have this but all I have at the moment is the cable coming from under a door frame with a 3 pin plug on the end which is too short , I was going to cut off the 3 pin plug, add in a length of cable then attach the plug 

As long as you can’t use a U/L listed extension cord, it’s just a simple power cable. Back before there were ‘short’ versions, I’d made several myself, by ordering a new cable, wanting to keep the original, and cutting it down to accept a store bought wall plug. Normal power care involved, but it really is just a power cable. I’d be a lot more concerned about jury rigging a battery to supply the correct power to a non-battery equipped speaker. 

These days, ordering one off of Amazon that’s close to the correct length is much easier, prettier when I look at it, and provides me a place to point fingers, when one fails….but after years, neither my own, nor purchased ones have.  Like I say, it’s just electricity. 

The only concern I’d have is I don’t know what ‘twin core’ cable is. If it isn’t standard power cable, don’t use it. Especially if it’s designed for speaker use (much lower voltage) or video use (again, substantially less power transmission). Use regular, every day, power cable. Available everywhere. 

Thanks for your replies , as you can see in the pic the power cable is about a foot too short , if I buy a Sonos one SL power cable and cut the length I need out of it then cut my power cable and solder it to my wire then that should be ok ......what dya think ???