Running >repadmin /options <Main DC Name> showed that both inbound and outbound replications were switched off, I switched them back on and my new DC received an rID Set almost immediately.
To reset INBOUND/OUTBOUND DC replications and ensure that they are switched on run the following commands:
>repadmin /options +DISABLE_INBOUND_REPL <Main DC Name>
Useful commands for domain controller testing:
You can have as many DCs as you need on an SBS network as long as the FSMO rID Master role remains on the SBS server.