Very little heat from heater - 2008 E 220 CDi

[JohnD25]

Hi all, any help or ideas appreciated.
I noticed this afternoon that there's very little heat coming from the heater even when turned up to maximum.
It was fine yesterday.
The engine is running at around 80 degrees (1 hour run on motorway and A roads) The coolant reservoir was full to the brim when I got home and the coolant was hot but not boiling so it is reaching a temperature to provide heat.
Could it be the heater control valve? Would a failure in that cause the high coolant level? I haven't had the car long and can't say whether the coolant was already high. Getting it serviced this month so was l leaving it to be checked then.

Thanks

 

[Blobcat]

Give the duo-valve a good sharp tap with a wooden handle

 

[JohnD25]

Give the duo-valve a good sharp tap with a wooden handle

I'll give it try thanks.

 

[mioba]

80 degrees after an hour run is low....a 2008 car. Stat perhaps