I want to run a spur off an existing wall mounted above kitchen worktop area
socket (on the ring).

I plan to run the cable vertically down, sunk in the wall (metal shielded)
to just below the worktop, then run surface mounted horizontally in plastic
trunking, then back up sunk in the wall (metal shielded) to the new spur
socket.

Any problems with this plan?

Cheers, Darren.

I plan to run the cable vertically down, sunk in the wall (metal shielded)
to just below the worktop, then run surface mounted horizontally in
plastic
trunking, then back up sunk in the wall (metal shielded) to the new spur
socket.

Any problems with this plan?

None. It is overkill, however. As there are visible fittings, you do not
need the metal shielding. The plastic trunking is also not needed for
surface run cables, although it can look neater if visible. Also, you can
probably just run horizontally without bothering to drop down. Although
vertically is normally preferred, horizontal runs are common in the above
worktop kitchen situation, particularly as they help with voltage drop and
earth loop impedence due to the massively reduced cable requirements.

Christian.