Does any one know how I can find out what was the position of the zenith over a uk location, for example, at midday on the 13th of January 2013?
How accurate does it need to be? Zenith has a declination equal to the latitude. RA=7h30m is approximately overhead at midnight on Jan 13th so add 12 hours for middday. So 19h20m, +53 would be about right for a UK location on Jan 13th.
Planetarium software like Stellarium allows you to set your location and look at the sky on any date/time.