An amateur with web design problems, please help

Do you have access to a console which allows you to view the files you have on the server?
If so, you'll need to copy all the new files into the www folder (ensuring the old ones are overwritten or deleted), as the FTP upload simply places them in the root folder on the server. Therefore anyone visiting hxxp://mysite.co.uk will see the new files, as they are in the root folder, but anyone who visits hxxp://www.mysite.co.uk will see the old site, as the files in there haven't been amended. You have to remember that the www folder is a subdomain of your site.

Well.... I suppose it could be that there won't be two folders. In my own experience, any of the host servers I've used have had a root folder, and a separate www subdomain folder.
Trial and error found that out, as I had the same problem as OP. Lots of head scratching later, and actually reading up on hosting gave me the answer.

Realistically, you could just have the index.htm file in the www folder, and have it point to the root folder for browser requests. That means only one set of files, and less space used, although unless you're hosting video or music content, that's unlikely to be an issue. It would also require you to recode some directory paths in that doc.

Does your FTP client allow you to specify the upload folder, or just the server? You could set it to upload to the www folder, and problem solved.

When you look at file manager, you should have the same files in the root folder (what you see when you log onto the admin console, and go to file manager), and in the www folder.
By default, the site should open on the index page, as that is how the server is programmed to operate. That being the case, you should have no need to type hxxp://www.mysite.com/index.htm to open the site, only hxxp://www.mysite.com, or hxxp://mysite.com
Check which folder is operating correctly. If you need the www subdomain included in the url, then you need to copy the files from there into the root folder, and if only hxxp://mysite.com is loading, then copy the files vice versa.

Btw...if www.mysite.com just opens a blank page, then you don't have the index file in that folder, or it's named incorrectly.

And remember that any files linked to (such as jpegs, gifs etc) need to be in the same folder as that particular page calling them, unless the path is to the specific folder they are in.

e.g. the logo on your site. if the directory path calling the image is simply [html]<img src="logo.jpg">[/html] then it needs to be in the same folder as index.htm

If however the path is something like [html]<img src="mysite/images/logo.jpg">[/html] then the jpeg image needs to be in the images folder.