In the novel Why Didn't They Ask Evans?, Inspector Williams is a police officer in Marchbolt and an old acquaintance of Frankie Derwent ever since he had helped her track down a maid using false references who had stolen some of her jewellery.
Frankie went to see Williams for some background information on the "Alex Pritchard" case. Williams told her that Prtichard's pockets had nothing which could identify him and that there was only the one photo of someone. This someone turned out to be his sister who identified him. As for the poisoning of Bobby Jones, Williams noted that the a car with the number plate number of the Bishop of Botolph had been reported in the vicinity but the Bishop's car never left his palace that afternoon, so it must have been a fake number plate.