Dear Stuart,
Sorry for my late response.
I have kept monitoring the issue these days, I just send three test emails to www.vitalizeconsultancy.co.uk/.../Events.aspx today, and all emails have been sent successfully withour error. Can you please check whether there are emails contents including "test from Microsoft" in your mailbox.
Would you please kindly test and let me know whether issue existed.
Can you please also kindly check whether "Alert me" feathure working properly in your SharePoint site?
As we know all emails send to users from SharePoint is from SharePoint mail system by default, I would like to confirm whether the mail system in your SharePoint site working fine or not.