Thanks both for your inputs. Very helpful.
I hadn't seen Laird - so I will look more closely.
So by my maths then a 10B packet (+7 overhead?) gives you 1 class A tx/rx every hour for ten years on a coin cell. This is with a bit rate of 250bps.
I did have one question - does the sensor need to join everytime it wakes up? What about if I completely power off the radio - I note the microchip part has errata.
Anything that transmits only 17B in both directions an hour is going to be somewhat low power, but the key thing is to consider the receiver sensitivity. If the application is useful with datarate then its impressive. Fortunately, there a a few good applications.