Will Watts

Thanks both - I've quickly realised that there was an unexpected .htaccess in my /admin/ directory that was restricting access to all files. I've renamed this, and the upgrade seems to have gone through correctly without any further issues.
Thanks for the info on the -TESTINSTALL - I'll add that now!

Thanks both - I've quickly realised that there was an unexpected .htaccess in my /admin/ directory that was restricting access to all files. I've renamed this, and the upgrade seems to have gone through correctly without any further issues.
Thanks for the info on the -TESTINSTALL - I'll add that now!

I think it would be helpful if you could post more details about this PHP error.

Please be advised that dev installations are limited in support. Please also be aware that with your license, you are allowed to run 1 Live URL and 1 Test URL. To utilize your Test URL, please update your installation in the ACP to use your license key with "-TESTINSTALL" (no quotes) appended to it.
You may wish to start over here as the 403 errors have me concerned that permissions on the server are not correct or ownership of the folders/files are not correct for you server. This can happen in cases of moving files around on your server and not an issue with the software directly. The CSS and Javascript for the upgrader are contained with the upgrade directory in your /admin directory so frankly, should not be having an issue unless there is a larger issue at hand on the server with accessing files.