I’m with fridge on this one. The original series 3 heater was pathetic. I now have a defender heater and it’s still pretty pathetic. The heat output is ok, but the blower output, by the t8me it gets up to the screen demise vents is woeful.
I have a mikuni version of an eberspacher - it’s excellent, and cost me less than £100 - gives I stand heat that is toasty warm in a hardtop series with no insulation in double digit minus temperatures.
I find that the 200tdi in mine never warms up fully when it’s really cold - after 30miles the radiator is still stone cold, and the temp gauge is below normal (and yes my thermostat is fine).