Right so - answering my own post, for the benefit of others who might want to go down this route:
Inside diameter = 12.54mm
Outside diameter = 15.44mm
I turned down a piece of plastic electrical conduit, slit it down the side and was able to install it over the rods in situ. A bit of brake cleaner to degrease, and a blob of epoxy resin either side and they're held in place nicely. Gear change much better.
Make sure the bushes, when installed, don't impede the full movement of the gear rods. I had glue on a 4mm protruberance at either extreme and that seems fine.