I "fixed" my issue by not using the Sage examples and just refactoring some choice bits of the Xero.Net.Standard OAUTH2 wrapper library...
I've yet to try and pull/push data (other than using postman) but i guess i'll be classing up some models for your data, I already have a bunch of SDO code I've been using for desktop that i knocked up about 20 years ago, are any of those fields/structures applicable to the cloud system? i.e. Account and Service Invoice?
Thanks!