Thanks @bennyling. You made me realise I have misunderstood the Tier 2 subscription. I had assumed alternate sources for the watch meant different to what is used on the phone. Whereas you suggest it is different to the default source offered by Carrot Weather.
So Carrot Weather will cost $7.99 upfront, plus $17.49 per year in order to be able to access BOM data on both phone and watch. That is rather pricey.