For anyone interested, I've managed to get a Win10 PC to connect to the RCM6650W-hosted ad-hoc network via a TP-LINK (updated drivers are important) WiFi USB adapter using the following article:
https://globalcache.zendesk.com/hc/en-us/articles/114094165352-FAQ-Windows-8-1-and-Windows-10-AdHoc-network-support-solution
It's not so straight forward as to simply click 'connect' from the WiFi network discovery list, but it could also certainly be worse!
Working like a charm to say the least.