My hosting company has informed me that today is (may be) the day!
The switch will require a freeze of the site while we copy and import the database. While this is happening you will see the "Maintenance" page and will not be able to do anything on the site.
Once the database is transferred, the DNS needs to resolve to the new server. This takes anything up to 24 hours (but is usually a matter of minutes).
Once the DNS is resolved, your browser will take you straight to the new site (which will apear exactly the same as the old site!).
Then I can start sorting the image upload, PM and white page issues.
Steve