Grant User permission to view XE results via sys.fn_xe_file_target_read_file
I have managed to do this now thanks to this article the missing piece of the puzzle was to copy the certificate to the master database

DECLARE @Cert NVARCHAR(4000) =
         CONVERT(NVARCHAR(4000),
                 CERTENCODED(CERT_ID(N'CodeSigningCertificate')),
                 1);

EXEC (N'USE [master];
CREATE CERTIFICATE [CodeSigningCertificate]
FROM BINARY = ' + @Cert);