I need to run some cat-5 cable in an old stable converted to an office.
Unfortunately the only way to run it is along a blank wall which is
Magnolia with a dark brown wooden skirting board. The floor is dark
carpet.

I can find D-line 'wood effect' trunking which is much too light, or
plain square brown PVC stuff. Neither of which is really neat enough for
the room which is done to a high spec and used for meeting (and hopefully
impressing) clients.

Does anybody have a better idea of how to run the cable unobtrusively?

My best shot so far is to make new skirting with cable runs along the
back, but that could be fiddly as it's an unusual custom moulding. I
don't really want to mess with digging into the walls if I can help it as
there have been damp problems in the past.

Wireless networking is a non-starter for security and speed reasons. WPA
is not good enough for this job...