MySQLRouter on Windows 10 Unable to connect to the metadata server
I think I have solved it. 

It appears that there is a difference between Group Replication and Innodb Cluster that perhaps needs to be made clearer in the documentation. 

Anyway, I ran: 

mysqlrouter --bootstrap root@[::1]:3306

and got: 

Error: Expected MySQL Server '::1:3306' to contain the metadata of MySQL InnoDB Cluster, but the schema does not exist. 
Checking version of the metadata schema failed with: Error executing MySQL query: Unknown database 'mysql_innodb_cluster_metadata' (1049)

This was the first clue that something was missing. Then I eventually found the following: 

https://dev.mysql.com/doc/refman/8.0/en/mysql-innodb-cluster-from-group-replication.html 

This tells you that you need to "convert" group replication to an Innodb Cluster by running: 

var cluster = dba.createCluster('prodCluster', {adoptFromGR: true});

This tells you that: 

A new InnoDB cluster will be created based on the existing replication group on instance 'localhost:3306'.

After that I tried bootstrapping mysqlrouter, and it worked.