van not starting on cold mornings?

I've a 2000 transit tipper. It has bother starting on cold mornings. Last winter I burned out the starter, probably because of the starting problem. I replaced that. Then I changed the oil and filter after I washed out the engin with diesel. The old oil was very heavy and thick. Now with cold weather coming back it is slow to start again. Is it heater plugs that's causing the bother?