^ Yup. I have read about this, its a known issue. And while it may not be the OPs issue, its often misaligned as a blend door issue. If you search the net for Discovery 3 clogged heater cores you will find many results. RRS also. Same design. The consensus seems to be they just dont flow fast enough to get the sediment back up and out the outlet pipe. The inlet/outlet are top and bottom. So there is a bit of a circular flow pattern which causes the sediment to end up more on one lower side than the other, which happens to basically be the bottom part of the driver side.

When Heat Fails Us - JE Robison Service - Bosch Car Service Specialists ? the blog

Preventative can help. Flush the core both ways. Citric acid (you can get it in powder form) helps. I have done this on my Benz and BMW was great results. Its just unusual for such a modern car to have this problem. A future project for me will be to install an inline filter for the core.

 

Had the same problem on my 2007. Took a couple tries by a shop back flushing the heater core with compressed air and water. Improved drastically afterwards but drivers side would start to blow cold again after a few months. This led me to assume it was a plugged heater, never checked the blend door though.