It stopped working after a few days. No dust left anywhere, I suspect that it's an electronics failure. Not happy with the whole Dyson design - difficult to clean, complicated electronics for a simple task. Pity, the heater had been convenient until now.

I had success with simpler steps
1 - Cleaning the top part, after removing the fixing bolt as described in steps 2 and 3; and
2 - Cleaning the bottom part, after unscrewing the four T8 screws located beneath the fan core as described in step 4.
The steps above are very much the easiest, because they don't require cutting anything or dealing with cables. I was able to vacuum out a lot of dust, and now the heater is working normally.