I just did a search in The London Gazette and found that Jacques or James Champ mentioned in the advertisement in the Sussex Advertiser was a solicitor in Chichester. He and "Mr. Clark, Solicitor, No. 35 Southampton-Buildings, Cancery-Lane, London" are mentioned together in the announcement (The London Gazette 8th April 1815 No. 17001). This Mr. Clark must be the same person as John Clark. This answers my question. Thank you for your trouble.
