This may help:
http://www.sheldonbrown.com/tyre-sizing.html
Short answer is 700c is 622mm diameter, while 27x1 1/4 is 630mm
@vikingfixed started
London Fixed Gear and Single-Speed is a community of predominantly fixed gear and single-speed cyclists in and around London, UK.
This site is supported almost exclusively by donations. Please consider donating a small amount regularly.
This may help:
http://www.sheldonbrown.com/tyre-sizing.html
Short answer is 700c is 622mm diameter, while 27x1 1/4 is 630mm