12thc. nave with aisles of seven bays, the W bay curtailed. The clerestorey dates from the 15thc. The one-bay square vaulted chancel originally had aisles, and remains of the S aisle are visible on the exterior. On the N there is a later vestry that continues the line of the nave aisle. The tower is set at the S of the nave's W end, and its erection dates from 1672. The building history is complex and is sketched in section VII, but here it must be noted that the building was originally the hospitium of Ramsey Abbey, and had no W tower. The nave arcades were shortened by just over one bay when the tower was added, and the tower arch appears to have been constructed of parts of the removed arcade including the W respond capitals. Meanwhile the original W doorway, of much the same date as the nave but stylistically unrelated, was presumably moved to its present position as W tower doorway. This is only one of several hypotheses that could be advanced to account for the present appearance of the W end. The tower and the lower parts of the aisle walls and the E façade are of ashlar, while what can be seen of the rest of the chancel is of cobbles. There was an extensive restoration in 1843-44. 12thc work described here comprises the chancel vault and arch, its E windows and the remains of the S chapel; the nave arcades, the W tower arch and the W tower doorway.

The town of Ramsey is situated on what was originally an island surrounded by Bury Fen on the south and Stocking Fen on the north, and was approached by a causeway on one side only. The abbey stood on the highest part of the island. The foundation of the abbey is attributed to Ailwine in 969, and the first mention of its leugata or banlieu (the privileged area around the abbey itself) is in a charter of Henry I of c. 1100-02. Until the end of the 12thc., the town was quite unimportant; it is not mentioned in the Domesday survey of 1086. By the end of the 12thc. it had grown enough for the Abbot of Ramsey to obtain the grant of a weekly market. There is no precise foundation date for the church described above, but the hospitium which became St Thomas a Becket's must have been founded c.1180 and seems to have been dissolved before 1291, when the first reference to a parish church occurs. Before this date the parishioners had the right of worship in the abbey church. The dedication at this time (or possibly in 1237, when many churches in the diocese were consecrated under papal pressure) was not to Becket, but (according to Cole) to SS Mary and Benedict.

The nave originally had eight bays. In 1537 the last Abbot of Ramsey, John Lawrence, ordered the executors of his will to make a payment towards the building of 'a Stepule in the Parish Church of Ramsey', but the result of his generosity seems only to have been the construction of a wooden belfry. This fell down in 1672, and the churchwardens then built a new tower with stone from the abbey. It has been suggested that one of the abbey towers was used for the purpose. The present tower arch was presumably constructed at that time, re-using responds from the curtailed nave arcade. Whether the W tower doorway is the original W doorway of the hospitium or an import from the abbey or elsewhere is a matter of conjecture. Stylistically it has no connection with other sculpture in St Thomas's.

A point of interest is the range of pier forms in the nave arcades, which Pevsner describes as 'a most instructive assortment of late C12 possibilities'. He also notes that the W doorway is more conservative in both its round-headed form and its sculptural motifs than work in the nave, which is certainly true and would point to it having come from elsewhere, although presumably not the tower in which it is set, which belongs substantially to the 13thc. The font is not included in this report since (and in spite of Black's assertion that it is earlier than any of the fabric) it is an early- to mid-13thc. piece, but this is of interest since it suggests that the church was in parochial use at that time.