We´re all doing mistakes in our everyday work, because we´re humans with emotions and different moods and not robots. But this can´t be an excuse for bad workmanship, which is completely contrary against the current regulations.
Building Regulations Part F (Ventilation), 1.2.17
"...All duct jointing,collars and components
should be installed to ensure appropriate duct
performance (duct runs should be as air tight
as possible). All joints should be sealed using
suitable sealants or tapes."
Building Regulations Part F (Ventilation), 188.8.131.52
"Units should be installed so that the
occupant can easily access them for routine
and regular cleaning of filters."
Building Regulations Part F (Ventilation), 184.108.40.206
"All duct runs should be straight, with as few
bends and kinks as possible to minimize
I need to say that the most installations, I´ve seen during my business activities, are well designed and installed. But unfortunately examples like the above ones do still exist.
As the occupier please take attention to such details, because they will affect your life, not the installers one.