I have now had a chance to read the very full account of St. Philip, Caerdeon, which accompanied its Grade 1 listing in 2018 (see below). It is indeed a building of exceptional interest because its architect, the Reverend John Louis Petit, was so exceptionally knowledgeable about European architecture, having travelled widely all over Europe and written two books to counter Puginian orthodoxy, advocating less strenuous restoration in order to retain the atmosphere of the original. I feel I should have known about him, but this is his only building: a building of great and original force, up a tiny road not far from Brithdir:

https://cadwpublic-api.azurewebsites.net/reports/listedbuilding/FullReport?lang=en&id=5249