The early history of the church is closely linked with that of St. Mary, under a single rector, and the phrase, 'the Church of St. John with St. Mary annexed, first used in 1400, is often found. St. John's was originally the castle chapel and was a 12th century, probably c.1130, church of considerable size and quality with a really ornamental tower over 65 feet high. At various times during the next two centuries the church was enlarged and in 1392 there was a chantry to St. Leonard. The western part of the church was rebuilt in the 15th century and by 1489 there were four altars. There was much ornamentation in the church and many chapels.
The first major restoration was in 1844 but by the late 1850s the nave arcades and aisle walls were leaning outwards to a serious extent. This resulted in a large scale restoration in 1862-3 with the nave extended by one bay, which added 154 free sittings, the west wall rebuilt in 15th century style and much else repaired and renewed. There were many other repairs and restorations to different areas between 1894 and 1924. Today the church has a chancel with north and south chapels, a crossing tower, transepts and an aisled nave with north and south porches.