One of the more civilised aspects of Sim courses at Heston was that sessions were two hours long divided by a two hour break.
From a training point of view this was infinitely superior.
a) Much easier to brief for 2 hours rather than 4.
b) As the average span of useful human concentration is about 45 minutes, the crew generally got more out of each session.
c) The 2 hour break gave time for a more effective de-brief/re-brief as well as an opportunity to relax and take advantage of the above-average catering facilities mentioned in the thread. Particularly valuable if someone was having problems.
With the merger of BEA/BOAC, training effectiveness was sacrificed for the convenience of standardisation on the industry-wide 4 hour session.