King William IV and Queen Adelaide paid a visit to Lewes in October 1830. Gideon Mantell was on hand to record the event, which he did in a small booklet that he published after the event. This 46 page booklet was entitled A Narrative of the Visit of their most Gracious Majesties William IV and Queen Adelaide, to the Ancient Borough of Lewes, on the 22d of October 1830. Published in London: Lupton Relfe, 1831.
A painting depicting the event has been hanging on the walls of Lewes Town Hall Assembly Room until the beginning of February 2017. It has been removed for professional cleaning and is due back in the summer 2017.See news item about it here
We can glimpse Mantell as one of the invited guests who attended the event and wno sat for a portait to be added to the crowd scene (depicted next to the uniformed man who was Sir T Downman and a child). He wrote in his diary
“All week employed as usual! Today sat (or rather stood) to Archer the painter who is employed on the picture intended to commemorate the King’s visit to Lewes.”