Strange 404 errors

Hi all

Recently been checking my site in webmaster tools and seem to have some strange 404 errors showing.

Firstly, Google says I have 404 errors on most of my manufacturer pages. Rather than being www.mysite.com/manufacturer webmaster tools is trying to find www.mysite.com/manufacturer-1. When you look at where it is linked from it says sitemap and a few pages of the site, but they don't have the "-1" so confused as to where Google is picking this up??

Secondly, I'm getting a lot of pages with ?filter_tag=manufacturer. Can I just robots.txt out the filter_tag? If so how?

Thanks in advance for any guidance.

Regards

Andy