I have already been using the latest build (2.3.5) since I noticed it last month. However, after several uninstalls and reinstalls of both the extension and Dreamweaver (which didn't fix the issue) I decide to try rolling back Dreamweaver to the previous version (19.0.1) and the Email Server Behaviours now work.
I have since spent the day migrating all of the Email SB's ready for uploading and testing but now my broadband and landline have gone down and have been told it won't be fixed until the end of the week.
Thanks again for your support and I'll add to this thread (probably next week when the broadband is fixed) if I run into any more difficulties with the Email Server Behaviour.
Many thanks.