What has happened to Oxford's retired emperors?

A hunt is on to find up to seven statue heads that once stood outside Oxford's Sheldonian Theatre,

The figures, known as the Emperor Heads, were first commissioned by Sir Christopher Wren in the 1660s.

The current set is the third to have been carved and it is thought as many as 27 retired emperors could still exist.

A team from the University of Oxford has located 20 but wants to trace them all to study how they have weathered.