Historic England From the Air – https://historicengland.org.uk/images-books/archive/collections/aerial-photos/
Historic England have made their archive of historic aerial photographs available online for free viewing. The above link will take you directly to the site
The most important Historic Places in England are listed, including buildings, battlefields, monuments, parks, gardens, shipwrecks and more.
A number of images of Sleaford are included in the archive and make for fascinating viewing. Give it a go!