If you have chunks like that falling out of your boiler after 10 months you have some extremely hard water. You will probably need to descale every 6 months. You would be well advised to look into a softening system for your equipment. It won't last long under the current conditions.

I should have explained that I have a double filter under the sink that takes out fluorine and contaminants down to micron sizes. So I am thinking that the gunk I saw coming out was put in either during manufacture, or during testing at the factory. So this may explain the visible stuff. What my filters don't do is filter for limescale. So if I get visible gunk next time I'll know that it's hard water.

The overall improvement in the machine's functioning has surprised me. I had guessed that, at best, the flush would do no harm and maybe improve things a bit. However the machine is now quieter during warm up; the pump runs much more smoothly, with no stuttering; the solenoid fill valve is now unblocked; the pressurestat is no longer sticking - and only the vacuum release valve remains sticky. So I'm delighted.