Am I going to be lucky.....
This blog was reporting that the transaction log was full and nothing could be posted. As with anything to do with SQL Server, there is a huge amount of documentation to plough through and Google searches didn't seem to show up a solution quickly. I couldn't even backup the database, as that was what was showing me the error message. So I tried the Import and Export Data tool. This is a great tool just copy to a new database on my local PC, create a new database using my hosting control panel and copy the data back. Then edit the web.config and hay presto it's working again...
Well it will be if you see this post!
Well, it seems that it was, but there were two issues. The second one what that the hosting company had enabled .Net v4, and this resulted in the error message, "A potentially dangerous Request.Form value was detected from the client". Another quick Google search gave the answer, add the following "
<httpRuntime requestValidationMode="2.0" />" to the web.config.
And it turns out there was a third issue. One caused by my copying of the database above. I created a blank database and copied in the data using the Import and Export Data tool. Well, it sort of worked, but there were various SQL errors trying to post to ID fields. So I guessed I should have created the database with the BlogEngine sql script. I did so, but then the Import and Export Data tool wouldn't move the data. To my rescue came EMS's Data Comparison and Synchronisation tool, http://www.sqlmanager.net/en/products/mssql/dbcomparer. I pointed the tool at the two databases and it miraculously did the job.