Webuser Forums: Why can some people not see my current site when others can?

I need help, before I tear the last of my hair out!

I've got two people that are having trouble viewing an up to date version of my website (www.portafold.co.uk). Both these people are only able to view a version of my website which is months old. (The third link down the left hand side should read 'message board'). I make changes almost daily but what they see is from at least 2 months ago.

They can't see anything that's reletively new and that definately do exist and even if they type in the full address to a specific page into their browser it comes up with a '404 not found' error.

I have cleared their cache on their PC's and removed any cookies and any trace of their surfing history from their browsers but it's still the same. One is using Firefox, one is using IE6. One is on Tiscali, one is on Wanadoo.

One of the pages they can't even access is www.portafold.co.uk/forum, it exists and other people can view it fine, but these two people get a '404 not found' error on their respective browser.

What can it possibly be? I've tried everything I can think of and I've been searching the net for an answer but to no avail. Following advice from Google I've added the line <META NAME="ROBOTS" CONTENT="NOARCHIVE"> into my <head> section on my index.html page to see if it was a problem through Google but still no difference.

Any help would be most appreciated.

Regards, Matthew