Fantastic. Thanks so much for persevering with this and looking into this for me.
I've written a Windows Powershell script which goes through all of the XMP files in the directory and removes the incorrect timezone info from them, which has fixed the problem.
To fix my files in Lightroom, I wrote all the metadata info from Lightroom, ran the script and then read the metadata back into Lightroom again and it is all running perfectly now.
Since no one else is posting about this issue, there is probably no point in sharing the script, but here it is anyway.
# Script to correct faulty XMP Date information in photoshop:DateCreated field
#
# Update 'path_to_directory' with the path to the folder that has the xmp files that you want to update.
# Note: the script updates all XMP files in that folder and in any subfolders as well
Get-ChildItem "c:\path_to_folder" -Recurse -Filter *.xmp |
Foreach-Object{
$content = Get-Content $_.FullName
$content | ForEach-Object { $_ -replace 'photoshop:DateCreated="(?<date>\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2})-.*"', 'photoshop:DateCreated="${date}"' } | Set-Content $_.FullName
}
