Hi,
Good news: After lots trial and error I've now got DNN 4.8 installed on my host via ftp and Plesk 8.1.1 
Bad news: The biggest problem turned out to be a known issue, but neither google nor searches on this site turned up any info 
Hopefully this post will get picked up by the search in future, so others can find it.
The problem was the following exception:
00:00:08.625 - Creating Portal: My Website
00:00:10.437 - Successfully Installed Portal 0:
00:00:10.437 - Installing Optional Resources:
Server Error in '/portal' Application.
Data at the root level is invalid. Line 1, position 1.
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.
Exception Details: System.Xml.XmlException: Data at the root level is invalid. Line 1, position 1.
Well, it turns out that you can't install DNN if you have /portal/ in the path. In my case the Virtual Directory and the directory on disk were both called portal This has been reported as a bug, and marked as trivial/wont affect many. Yes, I may be in the minority, but it did cost me 6 hours of carefully rolling everything back, ftp'ing 20MB and trying again, and again and again... 
Anyways, for reference the full output from exception (I had debug="true" and CustomErrors="Off").
Source Error:
Line 95: Response.Write("<h2>Installation Status Report</h2>")
Line 96: Response.Flush()
Line 97: Services.Upgrade.Upgrade.InstallDNN(strProviderPath)
Line 98:
Line 99: Response.Write("<h2>Installation Complete</h2>")
Source File: D:\Inetpub\vhosts\tracknite.co.uk\httpdocs\portal\Install\Install.aspx.vb Line: 97
Stack Trace:
[XmlException: Data at the root level is invalid. Line 1, position 1.]
System.Xml.XmlTextReaderImpl.Throw(Exception e) +90
System.Xml.XmlTextReaderImpl.Throw(String res, String arg) +127
System.Xml.XmlTextReaderImpl.ParseRootLevelWhitespace() +1953906
System.Xml.XmlTextReaderImpl.ParseDocumentContent() +503
System.Xml.XmlTextReaderImpl.Read() +41
System.Xml.XmlLoader.Load(XmlDocument doc, XmlReader reader, Boolean preserveWhitespace) +145
System.Xml.XmlDocument.Load(XmlReader reader) +96
System.Xml.XmlDocument.Load(String filename) +160
DotNetNuke.Modules.Admin.ResourceInstaller.ResourceInstaller.InstallPortal(String strFile, Boolean writeFeedback, Int32 indent) +291
DotNetNuke.Modules.Admin.ResourceInstaller.ResourceInstaller.Install(Boolean writeFeedback, Int32 indent, String type) +961
DotNetNuke.Services.Upgrade.Upgrade.InstallDNN(String strProviderPath) +744
DotNetNuke.Services.Install.Install.InstallApplication() in D:\Inetpub\vhosts\tracknite.co.uk\httpdocs\portal\Install\Install.aspx.vb:97
DotNetNuke.Services.Install.Install.Page_Load(Object sender, EventArgs e) in D:\Inetpub\vhosts\tracknite.co.uk\httpdocs\portal\Install\Install.aspx.vb:344
System.Web.UI.Control.OnLoad(EventArgs e) +99
System.Web.UI.Control.LoadRecursive() +47
System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +1436
Version Information: Microsoft .NET Framework Version:2.0.50727.1433; ASP.NET Version:2.0.50727.143
Rich.