I might have known Tony Cross would have his eye on likely swift nesting places and true enough he had visited the church at Llanilar earlier and seen the same behaviour but the cracks do not contain nests or young as they aren't deep enough. Its likely they are prospecting for next year.
If anyone reading this lives in llanilar and /or knows the Church warden or even the Vicar I would be glad of a call on 01974 299166 or email to Bob at the blog as we could discuss artificial boxes of some appropriate sort.
Tony's blog Ruffled Feathers on 23rd July features close ups of swiftlets and nightjarlets showing their comparable features and a glimpse of the enviable life of a Mid Wales Ringer!!