Duke of Marlborough

The Annual Register for the year 1758 (pp. 121-127.) contains an account of the circumstances connected with the trial of one Barnard, son of a surveyor in Abingdon Buildings, Westminster, on a charge of sending letters to the Duke of Marlborough, threatening his life by means "too fatal to be eluded by the power of physic," unless his grace "procured him a genteel support for his life." The incidents are truly remarkable, pointing most suspiciously toward Barnard; but he escaped. Can any of your readers refer me to where I can find any further account or elucidation of this affair?