The problem is that I'm using the latest version of Eclipse (Neon). Yocto only has plugins for Neon for versions >2.2 and IIRC I'm using Yocto 2.0.1.
The plugin for Yocto 2.2 works fine in that I can build projects but there's no sign of the ADT support.
I'll see how I get on with the GNU Arm Eclipse plugin approach and if that's no good, get an older copy of eclipse.