I thought if I formatted a cell as [h]:mm:ss, I would be able to enter  
57:00 into a cell

but it changes it 02/01/1900  09:00:00 (whihc is I know 2 days and 9 hours  
from base date)

It displays as 57:00:00

In fact I get exactly the same if I entere 57:00 into a general cell

Obviusly 23:00 works fine, but there must be a way of storing a time  
duration over 24 hours