The casework incorporates parts of the case which used to house the Choir Organ, added by Seede at the end of the 18th century. It was bracketed out behind the organist's seat when the organ was on the screen across the choir. It was known as a 'chair-organ' because the organist had to turn round on his seat to play the organ behind him.

In the reconstruction of the 19th century it was given away and used as a bookcase in a gentleman's library in County Durham. It was discovered by Mr. H. P. Chadwyck-Healey, who also generously paid for the building of a new instrument to go inside the case. The organ was built in 1956 by Harrison & Harrison. The re-ordering of the case was designed by Mr. R. H. Brentall. The stoplist closely follows that of the 18th century Choir Organ - a Mixture replacing the original Dulciana.