I have to hold my hands up and say I am not very good at cleaning regimes. I very rarely back flush or drop the shower screen. The water tank gets done every few months, but as it happens, I did just that today. Yes, there were loads of black floaters in the bottom. I realise that if it gets drawn into the boiler it will not last long. So, far from asking anyone else to hold their hands up I hope my post might prompt others who follow my regime to do something about it!