This is the scheme that I followed in the end. http://webpages.charter.net/tidmarsh/binmic/binmic.html
Straight into the sound card actually works great though as well. http://www.johncon.com/john/wm61a/
There are more complicated 'Linkwitz' mods and so on, but I don't think that we need that.
For mic-in the tip is the signal, and +5 V is given on the ring of the stereo plug. If you jumper these together in some way you can power the mic capsule without anything extra. Note that the contacts on the capsule are pretty damned tiny. Sacrificing an old pair of earbuds is the best move since the ends of the wires are small and already tinned.
I've been struggling tonight with trying to make a headband. Aiyaiyai...
Since the capsules are so cheap, I'll probably give this a go too. If I'd realised that decent capsules could be acquired for such a low price, I probably would have got on this sooner! Oh well, live and learn. Could you point me towards a schematic for the plug-in power box? I'm pretty hopeless with electronics if I'm honest.