Not sure if the l405 has a similar design, but I had the same issue on my LR3. It was a clogged heater core. My mechanic said he had never been able to flush them so said I was going to have to remove the entire dash to replace it. However I was able to fix it. I got a pump, connected 2 hoses to the heater core inlet and outlet, then had a bucket of descaling fluid that I let run through for 2 days. After that I attached a hose and pulsed water through it. That caused a lot of scale to break up. Worked great after that.