Why is it that this subject comes up each winter!!!
There is nothing wrong with Defender heaters they work great. Blow loads of air nice and hot out if and only if they have been adjusted correctly.
Mine gets so hot that i have to turn it down and thats in a Defender with van sides, unfortantly not so warm in the back due to the bulkhead. Thats why i put a aux heater i made in the back plumb up to the existing front one. Made it from a matrix and fan out of a Volvo 440.
Re poor heaters,
If its not blowing hot air on your feet then check the cables are still attacheed behind the speedo. Move the levers and have a look at the heater box you should see the cable ends move, if not the speedo out to have a look.
If it is but the air is cold, try bleeding air out the matrix thats all i have ever had to do is bleed the air.