Nissan NV200 cabin heater/blower

Does anyone find the cabin heater in a Nissan NV200 van is slow to get warm and never gets warm enough? I've driven several hours and it's still only warmish.

Is this a known fault or is the heater just not that effective?

Thanks in advance.

From frozen this morning, it took around 10 minutes driving to get warm, another 5 minutes later and I was toasty. 

Yours could potentially be the head gasket on its way out