My main riding partner in SC also has a Mercian fixed-gear, based on the King of Mercia but with cranked stays and cantilevers to accomodate fatter tires. He runs 39 and 44T rings up front and flips the rear wheel as it pleases him, and has been known to ride his on single-track, fixed.
The oldest Mercian I have ever laid hands on was this one <http://classicrendezvous.com/British...mp_Mercian.htm>, a 1950 Vigorelli owned by an ancient Englishman who brought it to the 2006 Cirque du Cyclisme back when they held it in Greensboro, NC. It was in hardcore vintage display form, complete with steel 27-in rims, a single GB sidepull, Brooks Swallow saddle, etc. My memory is that he also had the sprint carriers for the bike, the setup where you strapped your sewup wheels to your bars and a fitting attached to your forks so that you could ride your everyday wheels to the race, switch out parts as needed, race, then put it back together for the ride home.