If I tried to drop a cable down from above, and it hit a joist higher
than where I wanted the lightswitch, how should I tackle this? It
must be a common problem and I'm hoping it's the kind of thing where
there's a simple solution that electricians use all the time!

Use a stud detector to find the noggin. (These are the short horizontal
members. There will be one or two in the wall). If you haven't got a stud
detector, drill a hole near where you suspect it may be from tapping. Insert
a piece of stiff earth wire bent at a 90 degree angle. Twist the wire and
feel around until it hits the noggin and you can work out where it is.

Then make a hole (i.e. 4cm x 4cm) in the plasterboard just above the noggin.
Drill through noggin with a large bit, easily large enough to take the
cable.

To get the cable down there, use a piece of string with a plumb weight on
it. Drill a hole large enough to take the weight in the top rail from the
loft above. Drop the weight down inside until it hits the noggin, but keep
the string taught. Poke around for the string through the hole you made next
to the noggin with a piece of spare earth wire with a hooked end.

Once you have retrieved the weight, tie the cable to the string at the top
and pull the cable through. Have someone feed at the top whilst you pull the
string. Once you have retrieved the cable, feed it into the hole you drilled
in the noggin.

Now cut the large hole for the light switch, remembering Part M heights if
they apply to your building. Keep feeding the cable until it passes the
hole, or you can reach round inside to get it. Install a dry lining box,
install the switch and pull the cable back up to remove most of the slack.
Fill the plasterboard hole around the noggin with plaster. Repaint.

Christian.