When I post data to an html page on another website,
it comes up with a 'page not found error', if I hit
refresh the page then loads.

If I do a 'get' instead of a post so the data is passed
along the url it goes straight in.

Could anybody tell me why it doesn't work with form data?

There is the possibility of a reasonable amount of data so
I do not wish to rely on the url to pass data.