Hi Matt, 

Thank you so much for your response.

Well, I can say it's fixed although I can't say I'm 100% certain as to why. After struggling with this for days I updated pm2 from 3.2.2 to 3.2.9, this caused some problems so I reverted it back to 3.2.2 and then noticed this was working.

I have since re-upgraded pm2 to 3.2.9 with no issue. 

Posting this in case anybody else gets stuck on this in future. 
Thanks