I got it all working again. I simply found the masters log and position number by going to the master server and typing SHOW MASTER STATUS;
I then used that information on the slave and did this.
STOP SLAVE 'MDB1';
CHANGE MASTER "MDB1" TO master_host="xxx.xxx.xxx.xxx", master_port=3306, master_user="****", master_password="****", master_log_file="mariadb-bin.000394", master_log_pos=385;
START SLAVE 'MDB1';
Then checked it was all working ok, then I changed back to using GTID
STOP SLAVE 'MDB1';
CHANGE MASTER "MDB1" TO master_use_gtid=slave_pos;
START SLAVE 'MDB1';
After that it was all back and running again.