poor starting with P reg 1700 TD Astravan deisel?

My van doesn`t start in morning unless the heater plug light is turned on at least 3 times, as the weather gets colder the longer it takes to start, the van seems to start ok if it has been on a run ealier during the day, I`ve been told the heater plugs are ok, the power also seems to be down as the top speed is around 70 mph.