Emmo, Matt's detail is good but first focus on the "access, manage" part.
Few Xara designs are ever going to hit 400MB of web space, unless you are host your own MP4 or large number of images.

If you open you design and Export Website to a local folder, this will allow you to check the size of your website.
Do the obsolete file appear in the index_html_files folder? If so, you probably have to find the associated <design file>_Xar_files folder adjacent to your design file and purge them from that.

You then try another Export Website and if it has lost the obsolete files, you can Publish Website and Xara will recover web space automatically.

Failing all that and you know the obsolete files are not being republished, you can manually delete them from your web space.

A link to your site will help us.

You can get free hosting and in cases a free domain name.
Otherwise you can shop around for a cheap hosting provider that would cost less than a shop-bought coffee per month.

Acorn