Ohhh Darby! You are correct! And there's no rules to say you can't look it up anyway, so well done. I never believed there actually were 21 arches, but ... its true! BUT, I am 100% sure the datestone says 1847 (its actually on the arch over King Street) - if anyone is passing, perhaps they could confirm the date.
See, I'll have you all knowing more about your town before I have finished!
Thinking cap on again now......