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Thank for the very quick response - you have picked my favourite solution. It will be interesting to hear other views and comments.

We are looking for the very best way of handling potentially thousands (50k+) of 301 redirects following
a major site replacement and I mean total replacement.

Things you should know
Existing domain has 17 years history with Google but rankings have suffered over the past year and yes we know why. (and the bitch is we paid a good sized SEO company for that ineffective and destructive work)
The URL structure of the new site is completely different and SEO friendly URL's rule. This means that there will be many thousands of historical URL's (mainly dynamic ones) that will attract 404 errors as they will not exist anymore. Most are product profile pages and the God Google has indexed them all. There are also many links to them out there.
The new site is fully SEO optimised and is passing all tests so far - however there is a way to go yet.

So here are my thoughts on the possible ways of meeting our need,
1: Create 301 redirects for each an every page in the .htaccess file that would be one huge .htaccess file 50,000 lines plus - I am worried about effect on site speed.
2: Create 301 redirects for each and every unused folder, and wildcard the file names, this would be a single redirect for each file in each folder to a single redirect page
so the 404 issue is overcome but the user doesn't open the precise page they are after.
3: Write some code to create a hard copy 301 index.php file for each and every folder that is to be replaced.
4: Write code to create a hard copy 301 .php file for each and every page that is to be replaced.
5: We could just let the pages all die and list them with Google to advise of their death.
6: We could have the redirect managed by a database rather than .htaccess or single redirect files. Probably the most challenging thing will be to load the data in the first place, but I assume this could be done programatically - especially if the new URL can be inferred from the old.

Many be I am missing another, simpler approach - please discuss

Scenario Dynamic page produces great results for the user but produces a long very un-user and un-search friendly URL http://www.OURSITE.co.uk/s/ref=nb_sb_noss?url=search-alias%3Daps&field-keywords=loving&x=0&y=0#/ref=nb_sb_noss_1?url=search-alias%3Daps&field-keywords=lovingthebead&rh=i%3Aaps%2Ck%3Alovingthebead Solution 301 redirect in .htaccess Fantastic - works a treat BUT after redirect the original long ugly old URL appears in the location field Would really like this showing the new short user friendly short URL What am I doing wrong? Thank you all. CB

We have bought into a really powerful search, very exciting We can define really detailed product based 'landing pages' by creating a search that pulles on required attributeseghttp://www.OURDOMAIN.com//search/index.php?sortprice=asc&followSearch=9673&q=red+coats+short-length Pop that in a link Short Red Coats on a previous page and wonderful, that gives a page of short red coats in price ascending order, one happy consumer, straight to a page that meets their needs Question 1 however unhappy Google right? Question 2 can we meet Google's needs 100% with a  redirect permanent in an .htaccess file E.G redirect permanent /short-red-coats/ http://www.OURDOMAIN.com//search/index.php?sortprice=asc&followSearch=9673&q=red+coats+short-length
Many thanks
CB