The base of the tower was the original porch. Its 3-light side windows have round colonnettes under shouldered lintels. The S doorway is cusped and heavily moulded with 3 orders of nook shafts, foliage capitals and moulded arch. The double S doors have elaborate strap hinges.
The polychrome interior is faced with red brick, but with black-brick patterning, including crosses, stars of David and plain banding, and pale stone bands. The nave arcades have round piers alternately with crocket capitals and square abaci in early French style, and moulded capitals. The arches have red and black brick and Bath stone dressings, creating a rich polychrome effect, in addition to which the arches have cut brick zigzags. The tall narrow chancel arch is similar but richer, with foliage friezes and trails, while its inner order stands on pairs of short marble shafts on corbels and with crocket capitals. Alpha and Omega are inscribed in black brick roundels to the spandrels. The clerestory windows have polychrome segmental rere arches and sill bands. The nave and chancel both have roofs of closely-spaced arched braces. The nave W doorway, of 1933, has a 2-centred arch and double half-lit doors.
The chancel has a marble floor reached up steps, added by Scott in 1919. The E window has a moulded rere arch on attached shafts with annulets and crocket capitals, and a hood mould with head stops.
The font has a moulded square bowl with round panels having relief carvings of a cross, Alpha and Omega and the IHS monogram. It stands on a thin square pedestal and outer rows of detached shafts which are alternately round and polygonal. The pulpit is dated 1919, is polygonal with blind Gothic tracery panels. The benches are plain. The choir stalls have panelled ends and front, and a canopied back beneath open Gothic cresting. The priests stalls beneath the chancel arch have similar canopies and cresting. The chancel has a reredos of the Last Supper, part of Scott's modifications.
Numerous windows retain stained glass. The principal and most important scheme is the 5 windows in the N aisle of c1901-12, one of which is signed by R J Newbery of London. The sequence narrates scenes from the history of the church in Wales. It begins with Christ calling for the word to be preached to the Gentiles, followed by: Caratacus imprisoned at Rome; St Illtyd teaching Padarn, Catwg, Teilo and David; British bishops at the Council of Arles in 314; St David preaching at Llandewi Brefi; the visit of St Augustine; Archbishop Baldwin preaching the Crusades; William Morgan distributing the Welsh Bible; the Boer War and the mission to Africa. The tracery lights show the arms of the 4 dioceses of Wales - Bangor, Llandaff, St Asaph and St David - and the archdiocese of Canterbury.
The nave has a W window commemorating the Great War, showing the patron saints George, Andrew, Patrick and David above Bible scenes of David and Goliath, Samson with the jaw bone of an ass, Moses, and the Walls of Jericho. The chancel N and S windows, both dated 1923, shows SS Catherine and Cecilia. The E window is entitled The Good Shepherd and has several New Testament Scenes. In the S aisle, at the E end, are 2 windows also with New Testament Scenes and both probably by Newbery: one, dated 1898, shows the Ascension and Crucifixion, while the other, of 1903, shows 'Suffer the Children' and Christ in the Temple. Other undated windows show the Nativity, 'Suffer the Children', with the Visitation of the Archangel Gabriel in the W window, and the nativity in the N aisle W window.