unable to insert images

Hello,
Please help, I am unable to add any new images to a site.

Using the visual editor, the ‘Upload/Insert’ button works and I am able to upload images to the library without any problem, but when I click ‘insert into post’ the ‘add media’ popup overlay box just shows a page with the message ‘Apologies, but the page you requested could not be found.’
here’s the addrss of the page, obviously it’s an admin page so you can’t actually view it…
http://winscombemontessori.co.uk/wp-admin/post.php

If I try using the html editor and the ‘img’ tag button pretty much the same thing happens, except there’s no overlay ‘add media’ panel so I add the img src tags etc and when I click ‘Update’ I am redirected to the same Not Found page as already mentioned.

If I try to edit just the text in an existing post with images the same happens, although I seem to be able to edit and update the text on existing pages without a problem. I cannot add new images to existing pages however.
I’ve also tried inserting images that are hosted elswhere and have the same issue.

I’ve tried disabling all plugins and reverting to both twenty ten and twenty eleven and still have the same issue.
I’ve also updated to 3.3.1 and the problem persists.
here’s the site:
http://winscombemontessori.co.uk

Thanks for reading, hope a solution can be found.
regards
Marcus