"I'll bet my bippy it's a blocked exhaust vent or the lint trap."

Usually, sIx. Vents. People don't clean them--and as posted above, it's no biggie to clean them. Takes 15 minutes on a bad day. However, unplug the danged machine FIRST, or we'll be coming to do CPR or get yer heart back in rhythm. Another cause is belts breaking and the motor getting overheated: bearings failing, etc. People often overload their dryers and overwork the motors. Clothes dry faster if there's fewer of them in the dryer at one time. Some folks put bath towels and sweaters in with cottons and then wonder why it takes over an hour to get the load dry when if they dried them separately, it would take a total of 45 minutes. Go figger.