Problem solved!

And no sooner had I posted the previous entry than I discovered the simplest of solutions!!

For those for whom this might be useful, here's what I did... it really was this simple.

Open Mail and go to Preferences. Go to accounts and locate the Outgoing Mail Server.

From the drop down list choose  and then simply add a new server.

Mail creates an account and then provided you have "use any available server" ticked for your outgoing mail, things should work.

And I thought it was going to be complicated!