Plugging Leaks In The Heater System

In my '87 One Ten I've got next to no heat (standard Landy I know lol). If I have the blower motor just ticking over its a good heat coming out the vents but if I click it up to full it just feels cold with a minor hint of warmness.

How have you plugged the gaps etc. is it a case of disassembling the box removing the matrix and reverse engineering so it doesn't leak?

Thats what I did with mine as all the foam seals between the matrix and the box to bulkhead connection had fell apart, tbh its not that much better!
I used foam camping mats to make new gaskets, but lately have been using the neoprene foam from ebay real good stuff.
What temp is the engine running at?

It's a disco 200Tdi with the defender temp sender and it sits at half way on the gauge.

Cheers