Tim W wrote:

an aerial is just a length of metal I think

Sometimes with embedded voodo, the ones embedded within laptop screens
especially so.

Am I right in thinking I can leave the insulation on a length of copper wire
and use that? Will the length I cut it at make a difference? I mean
obviously it will be longer, maybe 30" instead of 3" but does it have to be
a multiple of a wavelength or something?

Usually 1/4 or 1/2 wavelength, but that'll be what you already have

http://www.fpvuk.org/forum/index.php?topic=1391.0

I am guessing the aerial can be
thin - a coat hanger not necessary here - but is solid copper better than
strands?

What's the velocity factor of a coathanger?

If you want to make existing access point more directional, you could
try this

http://www.freeantennas.com/projects/Ez-10/