Vivaro heater problem

Hi all, I have a 2002 Vauxhall Vivaro 1.9 DI which has a problem with its heater. The engine warms up to temperature but there is no constant hot air. It will blow warm temporarily but cools down quickly, almost like the water won't stay hot enough. The engine isn't losing water and there doesn't appear to be any air in the system. Has anyone had any experience of this / know of the cause? All heater controls are working.

Thanks in advance