How to work out the balance for an HD Wallet extended private key?
I have since found out that you can only use the bitcoin core to find addresses that are generated in the bitcoin core wallet.  You can add external addresses to the wallet, but then you need to re-index.  Code changes are in the works to add addrindex=1 to the config, which will index your local chain for all addresses.  Alternatively, you can use a block explorer website API service, or run Insight on your own chain.  So, no easy answer.  Probably best to use website APIs initially, and updated bitcoin core in the medium term.