
It turns out that the expiry for the Betas of SharePoint 2010 was this weekend, resulting in the web interface just not working any more. Microsoft don't support any upgrade path from the Beta either.
The options:
1) Use WebDAV (Windows Explorer, the path will be like \\siteUrl\DavWWWRoot\site\doclibrary) to access your documents and copy onto a new Farm
2) For List data you can access the data via Access and upload back to a new list or Access will try and create a new List for you.
3) The Content Migration API is apparently available, but I ran into issues using Central Admin to export data and import using PowerShell due to the differing versions between the Beta and the RTM on the two servers. Chris O'Brien has a tool to do just what is needed, but I couldn't get it working myself.
We didn't have much data, and I didn't want to spend money on a tool like Metalogix , so stuck with WebDAV and Access and it's actually worked out pretty well.
If we had more data and sites, I'd definitely be looking to a tool to do the heavy lifting for me.