XF 1.5 Googlebot Access Denied Crawl Errors


I'm a new Xenforo convert and have migrated one of my forums from phpBB 3.0.
I did a fresh install and used the importer. Everything appears to have worked well and we've been operational for a week or two now without issues.
I am using a redirect script so that the old URL's still work.
However I looked in on Googlebot and there are quite a few access denied crawl errors.
I've also tried one of the problem URL's as a guest and it does indeed ask for me to log in.
Guest users are set to be able to the view the forum and posts and the default permissions haven't been changed. I've checked them several times and they look ok, so I have no idea why it's happening.
URL that it's having a problem with is in the format http://www.myforum.co.uk/index.php?posts/20913/
Can anyone help?
Thanks in advance.