Hi Simon,

I have a portable supply here which uses a gel cell contained in a nice case
with carrying strap and the ability to plug it into an automobile cigarette
lighter or house current to charge it.  It has two female lighter plugs on
it and, I think, some outlets for some other voltages.  It goes by the name
of power station.  I bought it at Target here in Seattle and it cost about
$30 or so.  Very nice unit.  If you can't find it there you might try some
sporting good/outdoor camping supply houses.

Good luck.

Kevin, K7RX :)