He didn't have any particular interest in marathon running but was intrigued by the idea after discussing the event while visiting the Dysart Arms in Richmond one night.
Mr Dysart said the mussels living in the river were of "international importance" and had been wiped out despite the contractors being "well-aware" of the environmental sensitivity of the site.
Detailed examination has also compared the similarity between the dress worn by Susannah in the reworked version and that on Charlotte Walpole, Countess of Dysart, credited to Reynolds, which hangs at the National Trust's Ham House near Richmond.