Robert_Charlton - 1:14 am on Jun 25, 2013 (gmt 0)
Hi Serendipity - Welcome to WebmasterWorld.
The requested urls are getting 404 not found error (as shown in Google Webmaster). The number of these errors are decreasing though.
The 404s are expected behavior when you've dropped pages, and the fact that they're decreasing suggests that little by little they were going away. That's a good thing. 410s would be better, but it sounds like you don't know how to set them up on your server.
Also, to combat this I have installed a plugin which tries to locate any posts with similar words and redirects to that post before it generates the 404 error.
Bad idea. Get rid of it. Auto-generated tag pages are inherently Panda-unfriendly. Attempts to salvage them are only going to make this a bigger mess.
What is your worry about the 404s... particularly since you anticipated that they would happen?