Ah, think I have worked it out. Created and album in iphoto and called it videos. Then when to "photos" and searched for "movie". This appeared to find all the video files. Moved them all onto the new album. Then in imovie when looking in the iphoto library section one of the options is "albums" and there were all the videos ready to import. You would think that imovie would be able to automatically work out which were videos and which were still images, but this seems to work OK.