Typical a job just got bigger
Having to fit a 200x90 extract duct through an external wall (Timber
fame-cavity-brick)
Brickwork cut neat, cut inside dry lining fine ... removed insulation
and three are 2 single core cables (6181Y 1.5mm2) right through centre
of where duct needs to go.
Had no option duct has to go though a this point.

It is not an easy job to replace these - would need a large number of
8x4 dry lining sheets removed.
No option to remove floor above due to underfloor heating

I have drilled in a new hole in timber frame header (horizontal) to
allow reroute.

The is not enough slack in the singles to do this .... and easy solution
would be to cut each single and splice in a 12" extension. I can make
the cut so this happens above the header plate, and it can be clipped to
joist for mechanical support.

The splice ... is it acceptable to solder connect ? .,.... I could
solder, insulate & sheath with double layers of heat shrink, and
mechanically fix .... simple & neat.
Is this acceptable under 17th Edition ?

Not that any inspection taking place .... but prefer to do things right.

Alternative but bulky & not so easy is to splice in with 2 x Ashley J804
junction boxes.