Is there a way of disabling the annoying "Have a look at the online version" message when I open the Offline version?

Sorry no, you cannot disable the message about the new online version. If it's any consolation, you should find it is only shown once per week.

Regards

As I only use it twice a month (3 times when I do my VAT return) that's no real consolation. It's a bit like PPI calls-the more times I'm pestered....