Installation Error

I'd suggest repairing disk permissions and ensuring you're authenticating as an administrator who has full write access.  That error log looks suspiciously like there are places the OS is refusing to let you write to disk.  It's also possible that another install is in an incomplete state -- make sure you have everything possible shut down before running the Sophos installer.  Use Activity Monitor to quit all stray applications and processes.

Thanks for the response Andrew, however it's the same result I'm afraid.

I'd already repaired permissions, but I repaired them again and the only message I got was 

Warning: SUID file “System/Library/CoreServices/RemoteManagement/ARDAgent.app/Contents/MacOS/ARDAgent” has been modified and will not be repaired.

I then ran the installer again after a clean reboot (and remembered to uncheck the stupid Lion restart programs checkbox) but it failed again. In the /var/log/install.log I see this additional line, not sure if it's of any use

IFDInstallController 29516DF0 state = 7

My only other observation is as I go through the wizard, pushing next at the checking for updates step is very (very) quick, as if it is failing immediately. Possibly my imagination, and there's simply no update, however I thought it worth mentioning. Could a router cause any issues?

Sounds like permissions are fine... that ARD one is expected (Apple should fix the default IMO).

If the checking for updates step is quick, it's possible that your router is causing issues.  Are you allowing the download of installer bundles from Akamai?

I'm not knowingly disallowing downloads of installer bundles from Akamai !

Any way I can check?

If you don't have a managed router, you're likely not doing it.  Some proxies could cause issues too.

It's just a home setup with a Netear router and no proxy.

Did this get resolved?  I think I might be having this same problem on latest Snow Leopard:

Oct  6 00:09:15 watson-3 installd[244]: PackageKit: Install Failed: PKG: post-flight scripts for "com.sophos.ui.7.2.7.installer"\nError Domain=PKInstallErrorDomain Code=112 UserInfo=0x102a3dfc0 "An error occurred while running scripts from the package “SophosUI7.2.7.pkg”." {\n    NSFilePath = "./postflight";\n    NSLocalizedDescription = "An error occurred while running scripts from the package \U201cSophosUI7.2.7.pkg\U201d.";\n    NSURL = "file://localhost/Volumes/Sophos%20Anti-Virus%20Home%20Edition%20OS%20X%2010.4%2B/Sophos%20Anti-Virus%20Home%20Edition.mpkg/Contents/Packages/SophosUI7.2.7.pkg";\n    PKInstallPackageIdentifier = "com.sophos.ui.7.2.7.installer";\n}
Oct  6 00:09:15 watson-3 Installer[196]: install:didFailWithError:Error Domain=PKInstallErrorDomain Code=112 UserInfo=0x11937c5a0 "An error occurred while running scripts from the package “SophosUI7.2.7.pkg”."
Oct  6 00:09:15 watson-3 Installer[196]: Install failed: The Installer encountered an error that caused the installation to fail. Contact the software manufacturer for assistance.
Oct  6 00:09:30 watson-3 Installer[196]: IFDInstallController 632220 state = 7
Oct  6 00:09:30 watson-3 Installer[196]: Displaying 'Install Failed' UI.

I booted from the install disc and repaired my permissions first.  Despite the failed install, the s/w seems to be largely there.  On-demand scans seems to work, but it's not showing the Shield and did not install any Login Items.

You're likely going to have problems installing Sophos A-V HE 7.2.7 on the latest Snow Leopard.  Try downloading and installing version 7.3.3C.

Would love to.  Where would I find the image you're mentioning?

From the official website:

http://www.sophos.com/en-us/products/free-tools/sophos-antivirus-for-mac-home-edition/download.aspx