Comments not working

Hi,
I am unsure if this is the right place to post, but I wondered if someone could help.

There is no way to comment on my blog posts anymore, i.e. people can still fill in the comment, but when they try to post it takes them to a page that says

The requested page has not been found

I have been googling the issue for hours but cannot seem to find anyone with a similar problem.

My website is: http://melaniemartinphotography.co.uk (I have three domains pointing to the same site other two photographerbirmingam.org / melanimartinphotography.com )

What I’ve tried so far:
I have disabled all plugins, checked if comments were working… no luck
Reverted back to default setting for permalinks, checked if comments were working … no luck
Set Discussion settings as per wordpress.org default … no luck
Have amended .htaccess file in my web root – no luck
Talked to my host about the issue who are dumbfounded and have no advice to give so no luck (was told several times that it should be working

I am starting to wonder if there is issue with the recent wordpress upgrade to 3.3 and if I should revert back to a previous wordpress version, but I am not sure how to do that anyway.

Am totally at a loss and would appreciate your know-how… HELP