The Rix Uncoupling Tool is Designed to work with the Kadee® style Couplers. Place the Rix Uncoupling Tool down between the cars until the magnets rest against the rails, push one of the cars towards the other and the two magnets will cause the couplers to release.
The Rix Uncoupling Tool sells for about $3.00
This is the Rix tool that MMaD kindly set to me to try out.
It works fine on American stock where buffers don't exist or are widely spaced. I found that after a couple of tries, it worked better as the couplers themselves hadn't been worked much - opening and closing.
My efforts to get the wagons as close as possible have unfortunately made the Rix tool unusable with my British stock. The tool sits on the buffers and doesn't go down between them. I suppose that it was designed for American stock.
This doesn't deter me. I'll make one up a little wider and use some stronger neodymium magnets that I have lying around. I'll report back then.