Thanks for responding. Unfortunately it is not as simple as that. The images are not inserted into the post, they only appear in a gallery and because there are so many to do I do not use IDs as I would be continually adding to them through the month.
The gallery simply uses the attributes 'file', 'columns', 'order' and 'orderby' to display the images in the carousel. The images do not otherwise appear on the page or in to code of the page. They cannot be deleted from the page as they do not exist on the page and likewise they cannot be added to the April carousel (gallery) as they are already attached to March and cannot be detached.
My understanding is that you 'attach' an image either manually in the media library, or by 'inserting' the image into a post or page. Simply uploading the image to the media library does not attach it.
Uploading to a post or page both inserts and attaches the image. This is not what I need in this instance.
I need a method of detaching the images preferably other than deleting them and uploading them again, and not using the id attribute on the gallery as that is way too long winded for the hundreds of images I upload which are NOT suited to rearrangement by drag and drop.