Hi I have moved this forward and managed to find the groups I need. I decided to perform a small test to see if emails are received (group has 8 users). Message sends successfully but emails are not sent. If I send another message with say 6 users (ie not a group but just 6 individuals) then the email seems to be sent.
I am using WPMail SMTP plugin if that is relevant (which works when you test it).
Sending to groups is an important part of the site so I do hope we can resolve this.
Many thanks
Bill