[vjck_uk]

If you just take off the highest pipe on the heater and observe the flow through it.
If the temp of the engine is ok and the flow is good then it may be the heater matrix is not sealed around the edges inside the heater body.
The only other thing it can be is the heater controls are poorly or the felt seals on the flaps are knackered or foreign bodies (leaves)jammed in them so preventing them closing properly.
A small gap can result in a massive decrease in output temp.
Vaughan