My dryer in the last week or so has started to take a much longer time than usual to dry a load of clothes, what could cause it to do that?

In most cases when your dryer is heating and just taking a longer time to get the load dry that means the dryer has an airflow issue. Check the dryer exhaust vent hose ( the vent hose from the dryer to the vent on your home) and make sure it's not pinched or stopped up. If the dryer vent is ok then you need to clean the vent on your home because its stopped up. The reason it takes longer to dry a load when the dryer has an airflow issue is all the hot air being vented out can't vent so it backs up in the dryer causing the heating element to cycle on and off too fast making it take longer to get the load dry.