First things first.....
Is the the you are doing taking away an accessory?
Does that make sence? Umm.......
Once you have done the join, and it has been plastered over, will the cable be in a 'safe' zone, or 50mm or more below the surface?
A safe zone is:-
- Horizontally or vertically from an accessory, the zone being the width/height of the accessory. This zone extends to the other side of a wall if the wall is 100mm thick or less.
- Within 150mm or the corner of a wall, or where the wall joins the ceiling (still a dirty practice if you conceil wiring here!).
The only other option for T+E would be if it was in earthed steel conduit or trunking, or protected against penetration by nails and the like. (Steel capping from a DIY store or merchant WOULD NOT be adequate).
If your cable does not run true to above, you need to make sure you leave an accessory on the wall (A socket, switch, blank plate even).