On the Mk3 Gold Midas they use the metro heater assembly. This is quite reknown for having quite an inadequate heater, which is strange, because it works fine in a metro from my own recollections.
From my own investigations I found out that one of the main issues was the interface between the fan housing and the heater unit. These are joined by a special GRP duct, specific to the Midas. Unfortunately the connection is not very airtight and has too severe a turning for the air to flow, causing some of the airflow to back up.
The solution is to create an airtight duct between the fan housing and the heater unit and change the duct to something with a more gentle radius (like a large diameter flexi-pipe).
I haven't done the second option yet but to create an airtight seal on the existing duct is easy to do. Basically get hold of a wheel barrow inner tube. Cut a piece off about 6 inches wide. Undo fan assembly. Roll inner tube so that it joins both the heater duct and the grp duct equi-distant (so there are 3 inches covering each side. Job done. Air tight seal and more flow to your heater not spilling about in all directions.
Dont forget to rivet/glue a blanking plate over the output at top of heater unit. This is specific to the Metro and is not used on the Midas. It will just mean that excess air flow is directing to an area under the dashboard doing nothing.
Cheers,
David