I've just checked out my domain provider settings (123-reg) and I have these options:
1) No web-forwarding - If you select this option anyone typing ianhoughton.co.uk into a web browser will be shown the 123-Reg default page.
2) Framed web-forwarding to a single page - If you select web-forwarding to a single page the destination URL can have a specific filename on the end (e.g. index.htm). It could just point to a directory (URL ends with /) in which case the files displayed would depend on the webserver 'directory index' settings. The destination URL will be replaced by ianhoughton.co.uk.
3) Non-framed web-forwarding to a single page - This option is the same as above however the destination URL will always be displayed in the address bar of the browser.
4) Framed web-forwarding - The destination URL will be replaced by ianhoughton.co.uk
5) Non-framed web-forwarding - The destination URL will remain in the browser's address bar.
I have my main domain ("http://www.ianhoughton.co.uk"]) setup with option 1 and the nameservers pointing to my hosting provider. My other domain ("http://www.ianhoughton.com") is setup with option 5 and brings up the .co.uk domain.
I think as Steve has rightly said, the issue is with the options with the domain provider and not your hosting company
I hope this helps....