I'm not familiar with Sykes' book, but the fact that he makes statements of opinion on the subject of Foliate Heads (the churches of East Anglia, in particular, have a lot of these, all, so far as I know, carved in situ, few if any earlier than the mediaeval period) and the Sheela-na-Gig (some of which may indeed be old carvings incorporated, along with other salvaged building materials, into later constructions; a common enough practice) is not in itself evidence of anything in particular, and certainly not of any direct connection with earlier deities; such connections may well exist, but it's not an easy thing to establish!  Britain and Ireland contain many examples of churches built on pre-Christian sacred sites; again a common practice where a new religion wishes to supplant its predecessor(s) as painlessly as possible.

Malcolm