Yup - unfortunately ghastly price of £33.00 (ISBN: 1841718807). Does not seem to have been put on the BAR website yet (http://www.archaeopress.com) but it has literally only been out for about 2 weeks.
I know many Mendip cavers (especially Wessex Caving Club and UBSS) - you're all mad :)... I think the swallets are fascinating and archaeologically they seem to have been used as a focus for ritual deposition in the Neolithic, Bronze Age, Iron Age and Roman periods (have you seen the article I published in http://capra.group.shef.ac.uk/2/issue2.html. ?). I have heard a few weird stories relating to them today from speaking to locals (I used to live in Priddy) and certainly some were used as boundary markers in Anglo-Saxon and Medieval periods. Facsinating stuff.
Jodie