Thanks for the advice.
I have gone through the suggestions in Google, but they don't work, that's why I came here.
For example, I have tried to disable the theme, but that then stops everything working... all I see is a completely blank white page when I try any URL of the site (and still can't login).
I have tried to disable the plugins via both the database and FTP several times, and the plugins are still working. Very strange.
Any other suggestions please? I still can't login to our blog.