Absolutely correct SDQ, it is St. Cyngar's Church, Llangefni, also known as Eglws-y Plwyf, which is at the start (or finish) of the Nant-y-Pandy Nature Reserve Walk. We took a No. 32 bus out of Llangefni, got off at the Forestry car park just beyond Rhosmeirch (where we were amused to read the "flick-it-with-a-stick" notice about the dog-poo that Jack mentioned in the Great Orme thread recently!) and followed the path back past Llyn Cefni and alongside the old Pentre Berw-Amlwch railway line, which still has the tracks down and I believe is under consideration for a Llangollen type restoration. Took about an hour from getting on the bus to arriving back into Llangefni, and is another flat enjoyable walk. As you normally come up with extra info. perhaps you could tell me what big the works/factory is next to the dam across the head of Llyn Cefni.
Thanks also for your reply Hollins, I had absolutely no ideas about the Tenby church question, even after Hugo's extra clues!