could you not consider using a wireless shutter release instead? then you arn't so limited as to how far away you can go from your camera.
I'd use a 3.5mm plug/socket and then get a standard 3.5mm extension cable. Should cost less than £10 total. They do make some plugs/sockets that don't require soldering if you don't have the equipment/skill, but soldered is going to be much more reliable.Tim, depending on the number of wires involved, it's probably fairly easy to make an extension cord for your current wired release. Cut the release's cord in the middle and solder on a suitable plug and socket so they can be plugged back together then make an extension making sure that the pins all match up when it's all plugged together.