Thanks for all your responses they have really given me some food for thought!. having spoken to the other web site manager in our group of companies I have decided that a CMS would not be the way forward as due to certain factors in our company it is better for us to have full control over our code.
I think the way forward for me will be to put sections of the site into separate php files and then get myself some serious htaccess tutorials on the go!
There are a couple of options in these replies - can anybody tell me if it is better to use 301 redirects or a rewrite of the file names? Dr tandem your mthod sounds like it might be a simple way to add php functionality to our site.
In terms of the url structure i have recently changed all internal and what external links i can so that the structure is
www.mysite.com/productname/ however our leading product page which performs well in all SEs has a full filename URL it is this page that worries me in changing to php.
thanks again