2014 sprite major 6 TD poor fan speed

I have a Sprite Major 6TD 2014, just come back from a week away with the children, using the heating for the first time. We noticed that the fan speed can't be adjusted/sped-up. The rear of the van was always colder and very little blown air from the vent beneath the bunk beds. Does any on have any ideas please to help rectify. Or is this a design problem on this type of van. This is our first purchase. 

Thanks phil.