The middle arch was built over a sepulchral vault seven feet long, two feet wide and three feet deep, covered by large stone slabs: one of these was a stone coffin lid upside down, with a very old incised cross head, and one a large slab of Purbeck marble, fragments of which were found in various places. There is no connection between this lid and the vault. The vault was empty in 1878 but there were bones scattered around and signs of lead having been melted down. It is possible that Abbot John de Horsley's (1301 – 1338) slab which is now in the museum on the site of the Chapter House originally covered this vault. It had a marginal inscription in detached Lombardic letters, brass between 2 narrow fillets. Only 2 or 3 of the brass letters survived to Hope's time, but he read HIC: I... DE: HORSE... PICIETV...

With our backs to the eastern end of the chapel, let us progress forward, back into the south transept, and turn left down the remaining steps into the Sacristy. This was a small room divided into two, one half being called the Treasury. It had doors into the church and into the chapterhouse, the latter having been blocked before the suppression. According to the sketch made by Buck in 1727, there was a door into the cloister; this was not excavated in the 1878 excavation and could not be verified by Colvin, on the site in 1938, because it had all disappeared. Buck’s drawing shows a small pointed doorway from the cloister ambulatory, near the north end.

Let us return up the small steps, proceed to the crossing and turn left into the nave. It is a little difficult now to imagine what the nave used to look like. For one thing, only the east end has been excavated; for another, it’s probable that it used to extend further to the west than is currently accessible by permission of the owners of Abbey House; for yet another, the north side of the nave is now obscured by a rough area which the owners call their thicket.  

However, if we assume that the nave originally had three bays, it would probably have measured over 90 feet long, with its breadth the same as that of the chancel. This breadth incorporates a north aisle, which was apparently separated from the nave by three obtusely pointed arches supported on clustered pillars of a bold and effective design. Sir William St John Hope, the first excavator on the site, found three Early English arcade bases and left two of them exposed: they were 30 feet apart, and four semi-engaged shafts of their pillars had intervening bands of dogtooth.

One writer tells us that this north aisle was an afterthought, and another, that the lower parts of the walls date from the 13th century. The18th century sketches by Buck and Stukeley both show the nave with a clerestory and three windows each side: fragments of early Perpendicular tracery from these windows, which were square-headed with three lights each, were found in the excavation. The windows of the clerestory and the roof were built under Abbot John Spondon in about 1440.