I have no proof that Martha is the daughter of John Crutchfield b 1757 since I have not managed to find a baptism entry for her. However,

• Martha is living with John in 1841 and very close to step-brother John.
• When John married Mary Ball in 1782 the banns named him as John Scratchwell and the marriage entry itself reads "John Scratchwell of this Parish Bachelor and Mary Ball of this Parish Spinster..." but he signed John Crutchfield! When Martha died her death was registered under the name Scratchwell but the burial at All Saints, West Ham recorded her name as Martha Crutchfield.
• Martha was a witness at the marriage of James Crutchfield (John's son) to Grace Clarke in 1817.