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What I thought of doing instead was to move the whole site onto a new URL (widgets.com -> widgets.co.uk) and then just do a perm redirect on the home page of the old domain to the home page of the new page so that the PR and backlinks are carried across.
Do you see any problems in me doing this?
One thing to point out is that widgets.co.uk currently has a perm redirect to widgets.com.
Changing domains is fraught with challenges and potentially sever bumps in the road. Only do it if you really "must" I would say. Better if you do what you can to clean up the existing domain, or perhaps even redevelop there according to your new scheme. I also would not focus on 301 redirect for the whole solution -- where are those other urls coming from? Plug those holes first.
So what is happening is that the same actual page, but with different querystring items, is causing a duplicate page penalty. i.e.
So even if I change the mod-rewrite and make all these pages 401 I think I will still get a dup content penalty in Google because it doesn't seem to remove the pages from its own cache.
So as far as I can see my only 2 options it to either try and catch all pages with a 301 redirect or move to a different domain (which has been registered for a few years now but not used).
I have now gone for SEO urls.
So what was once:
(same data, different pages)
(same data, same page, different querystring)