Hi David,
Thanks for your prompt reply and email support. Just posting the solution here for the benefit of other users. In the WP Dashboard > Media > Settings menu my uploads url was set manually to wp-content/uploads/ instead of the default wp-content/uploads
I just had to delete this, save settings, and the restore worked as expected. David has also tweaked the next release of UpdraftPlus to remove any trailing slash when using the configured uploads directory, so that will also fix it into the future. :-)
Thanks for the great support!