New bathroom Installation

The problems you have encountered with a fan failing and a noisier replacement may be unconnected with the state of the ducting. Without photos it's not possible to comment. A good quality extractor should be quiet and will last at least 5 years if the blades and vents are vacuumed clean from below every few months. The state of the flexible vent pipe may be entirely apppropriate. 110mm flexible ducting is thin and duarble enough for the job it's intended for. It can be attached to the motor outlet by cable ties or by duct tape. The latter is less likely to detatch. I'm unclear what the "loft ventilation" you mention actually is. If it's an air brick or similar this may not be appropraite. The exhaust should feed directly through the eaves, the roof or a gable end to an outside grille. Insulation of extraction ducting is not a requirement and is in any event unecessary on a normal extraction run. The pipe can however allow water vapour to condense if the pipe run is long, vertical or horizontal without a fall to the outside. Water pooling or falling back into the fan/bathroom can cause motor failure. There are several ways to prevent this. More information is needed to advise properly. If in doubt engage a tradesperson who is familiar with extractor fan and ducting installation. Building Regs part F state: UK Building Regulations (Approved Document F) require a minimum extract rate of 15 litres per second (L/s) for a bathroom's intermittent extract fan, with the fan installed as high as possible, no more than 400mm below the ceiling. Continuous extraction systems have a lower required rate. Fans must vent directly outside, not into lofts or cavities, and all electrical work must comply with Part P Building Regulations for safety, especially when dealing with bathroom zones.