Pic by Jenny

The church was built by a Benedictine Priory which was founded in the town at the end of the 11th century. It was said to be one of the richest religious houses of Lincolnshire. The church was built by the Priory at the end of the 13th century on top of a previous church. It was completed in a cruciform shape around 1300. Originally there was a nave with aisles of six bays, transepts and chancel. In 1360 the south porch and tower were built and the church was enlarged with an outer south aisle. Alterations were carried out in the 15th century when clerestory windows were added, and the north porch and Gleed Chapel (north west) were added. In the 18th and early 19th centuries high box pews and galleries were installed. These were removed during extensive restoration in 1865 under Sir George Gilbert Scott.

The tower was originally detached from the church, but incorporated in the 14th century when the outer south aisle and south porch was built. The crocketted spire has alternating lucarnes and flying buttresses and is 160 foot high. There are 8 bells.

The nave is now of four bays with a north aisle and two south aisles as mentioned earlier. The layout of the church is quite complex due to the additions over the years. The nave has a hammerbeam roof from the 15th century with carved Angels. There is also a large brass chandelier from 1760 with 36 candles. There are several hatchments on the walls, with many to the Johnson family.

There is a rood screen which was originally mediaeval but was heavily restored in the 19th century. The chancel arch has a five light east facing window above it.

The chancel is Early English and on the north side there is a Victorian choir vestry and organ. The east window is a triple lancet, and there is a painted ceiling which was designed by Dykes-Bower in 1959. To the south of the chancel there is the 14th century St Thomas's Chapel which was one of the first additions to the original church. Originally the only entrance into the Chapel was from the churchyard. There was also an upper floor which was used as a school, but this has now been removed.

East of the north and south transepts there are small chapels. The north Corpus Christi Chapel contains an Easter Sepulchre/tomb recess. The south St George's Chapel has been designated as a war Memorial Chapel.

The north porch, dating from the 15th century, has a parvis room. There are some fine carved figures on the arch to the north door.