Change Connection Broker HA Connection String 2012 R2

We have a 2012 R2 RDS Solution running with a pair of HA Connection Brokers using a 2008 R2 SQL Server to store the connection database.

The SQL Server has recently been updated to mirror with a second server and I need to update the connection string on the Connection Brokers to include the new SQL Server as the Failover_Partner.

I've tried using the set-RDConnectionBrokerHighAvailability powershell command but it errors with a generic write error!

I'm wondering if I need to remove the second Connection Broker before trying to update the Connection String or if I can maybe change the string via regedit?

Cheers for now

Russell