Well, no idea what or how or why but it's working now.
I ran 'sudo ./dslstats' to see if it was permission related.
It loaded dslstats with a blank config.
I went straight to the alerts tab, entered my info and hit test.
The Event Log showed "Email alert sent: "Alert: Test alert".
Bingo, email received successfully.
I closed this instance of dslstas.
I then double clicked the dslstats icon, and selected execute in terminal.
My previous configuration was still there.
I went straight to the alerts tab. The email details field was already completed with the same details that didn't work previously.
I hit the test button.
The Event Log showed "Email alert sent: "Alert: Test alert".
Email successfully received.
No idea why it's now working. I set this up weeks ago and have made multiple attempts to get this working.
edit: looking in my routers logs i can see....
10/04/2018-21:50:49 TCP 192.168.1.122:51172 ----- 64.233.184.108:587
192.168.1.122 being my Pi
I assume 64.233.184.108 is Gmail.
There was nothing showing in the logs before, so something was preventing it even trying.
Cheers for the help Ned