The study of cross-slabs dates back to the 19th century, in recent years the leading expert has been Peter Ryder, whose work on northern English cross-slabs has established the benchmark for recording and study. He has published extensively on cross-slabs including several county-wide systematic surveys in the north of England. Along with Aleksandra McLean of York University he runs the Cross Slabs in Northern England project https://sites.google.com/site/crossslabs/about-the-project
For more information on Gloucestershire cross-slabs see Bris. & Glos. Arch. Soc. Trans. 91 (1972) pp 150-8, “The Cross-Slabs of Gloucestershire” by Ruth F Butler and Leslie J Jones, which is available as a free download from the Bristol and Gloucestershire Archaeological Society website http://www.bgas.org.uk/code/browse.php?vnum=91.
For a general introduction to cross-slabs see Ecclesiology Today vol 43 (2010) pp 29-44 'Abused, neglected and forgotten: the story of the medieval cross-slab" by Brian and Moira Gittos.