PS - Witton will know more about this but there might be an issue with measured versus certified.

Apparently anything that is measured is said to have an error (for arguement's sake say plus or minus 0.05km). What a certified course does however is add in a correction factor (in this case 0.05km) so that the "real" distance is 5.00km-5.10km rather than 4.95km-5.05km.

A course certifier will use one of those wheel things and ensure that the course is measured via the shortest route. Therefore nobody (in theory) can run the race in anything less than 5.00km. I guess this is why "certified" courses are often called "long" by runners...it is just the nature of certified road races...

God knows how the old Radcliffe 5K was ever certified though