I believe that the conditioning period for extract beer depends on the quality of the syrup. Extract is the resultant liquid left when malt is being tested and it can be made from poor or fine malt. To make it stable and easier to ship the liquid is reduced to syrup. The extract was originally bakers malt extract. The conditioning period for beer is dependent on original gravity, primary ferment, wort stability, types of sugar produced during mashing, body, protein, beta glucan and starch carry over. The more gunk ( albuminous protein, beta glucan and starch carry over which is caused when mash out is performed with single infusion) that is left in the wort causes the beer to deteriorate during conditioning. When the gunk drops out hop character and flavor suffer, leaving the beer thin and sweet. When the protein drops out, the beer becomes thin due to lack of body.
Marris Otter is an over modified malt which means that it is less rich in enzymes. (Kolbach or SNR number determines modification). The malt is low protein which is good. Actually, when it comes to making beer, other than single infusion beer the malt is extremely difficult to work with due to over modification.
Although, a recipe may contain several types of grain it doesn't necessarily make the beer complex. The complexity comes from variances of the malt dictated by nature. Due to variances caused by nature a malt data sheet accompanies every sack of grain. The sheet helps a brewer to determine the quality of the malt and its make up.