export video to camera roll

I've an ipad 3 and I'm trying to export a video file 52mins (about 3.85GB) from imovie to camera roll so I can export the video.

But it won't work...

I keep getting a popup message saying it has completed sucessfully but when I go to the camera roll there is nothing there (except the source video).

HELP!

If I look at the settings I can see that iMovie is the size of the exported file but it has not been transfered to the camera roll.

I've check the privacy settings and the camera roll has been shared with iMovie, so it doesn't make much sense...

I've used the ipad to edit files fine before but these have only been about 20-30 mins (>2GB) videos that have transfered without any issue.

I have recently updated to ios 6.1, could this be an issue? I seem to have ran out of options other than look for an alternative editing tool and tablet device and move off iPad (which obviously I'd prefer not to do).

Thanks,

Craig.