All three are remembered by monuments in the core of the church. On the north wall of the sanctuary is a marble bust of Matthew Boulton, set in a circular opening above two putti, one holding an engraving of the Soho Manufactory. This was commissioned by Boulton's son, from the sculptor John Flaxman. On the wall opposite, below a pointed arch, is a stone bust of William Murdoch, spelled with a 'ck' — different from his own spelling. James Watt was buried in the grounds of St Mary's, but expansion of the church placed his tomb inside the church. To the south of the sanctuary, in an austere and serene space built especially for it – The James Watt Memorial Chapel – is a marble statue of Watt by Francis Legatt Chantrey, who regarded his statue of Watt as his favourite.