I am trying to apply a function that works out the time worked, and then if
below 6hrs display as time, or if over six hours - deduct 30 mins before
displaying the time.

The feeder cells have been formated to time and it all seems to work - apart
from deducting the 30 mins. Here is what I have used (well one of many
attempts!)

=IF(O3-N3<=6,O3-N3,O3-N3-$U$1) [where U1 is a time cell containing 0:30:00]

Any ideas?