Can you post a larger image of the maker's mark? It looks to me like Frances Stamp of London. If so then then the mark is not often to be found.
Frances was the widow of James Stamp of 86, Cheapside, who died on the 11th April 1780. She entered her mark on the 12th May 1780, and it is likely that James Sutton took over the business as from the 7th July 1780, the date that he entered his mark from the 86, Cheapside address. If I'm correct, then your tongs can be dated to within that less than three month period.
Also, is the Lion Passant struck directly over the solder line?