I took a quick look at the site, and while it's best to have unique content, I don't think that is hurting you much, since you have pretty good PR.
I don't like the home page optimization much, I would have more general title, description and keywords there and not mention products so much and save that for the product pages, which you have done. I would use more keywords about the main topic of the site and describe your weight loss product site.
Now, the titles on the product pages all start with the same word for the name of the site, and this is a waste in my opinion. I would either remove that or move it to the end of the titles.
Your description tags for the product pages (I looked at 3) are good, but then it appears that you are using the same keywords tag on each page:
<.TITLE>WeightWorld – Botanical Detox and slim to revatilise your whole body online in UK.</TITLE>
<.META NAME="description" CONTENT="Detox and slim is herbal and natural weight loss product, Botanical Detox and Slim with Camomile is a detoxification and weight loss program for inner cleansing of your body. Detox and slim is stimulate your digestive system in a natural, safe and effective way.">
<.META NAME="keywords" CONTENT="Hoodia, Hoodia tablets, Hoodia medicines, hoodia details, hoodia advise, weight loss hoodia, medicines, tablets">
Not only is this wrong for the page, but I have seen some signs where this is can hurt the pages and be seen as "duplicate content".
In my option, Google considers duplication in title, description, and keywords tags as a sign that the webmaster is "lazy" and pays less attention to the page, Even if the the page content is fairly unique.
I ran a site scan on your site and it said there were a lot of errors. Some of these are not correct, but a number of them are and I think you will want to go through the report and fix the ones that really are errors. You can see the report here:
http://www.dp8.net/weightworld/.
Also, I found some CSS files that were not CSS files but had a message in them that said they were there for "SEO Purposes"??? I don't know of any reason you would do this, and I would advise against it. Still, if it works and you want to take a risk with it that is your decision.
I do think it's best to NOT change too many things at once. That way you can make some changes and wait a while to see what the results are. If you change too many things, then you won't know for sure which one helped or hurt the site. A way to test what you are doing is to make a change to one page or a section of the site and see what happens. Those with a number of sites can make different changes to different sites and see what happens.
One last thing, I see you have an XML sitemap which is great. But it indicates that your site is changing every day. If this is not true I would change it to reflect the average rate of change.