A question for the experts: is it possible to rig up a DIY powerflushing system for the central heating? My suggestion is:
Connect mains water supply to a drain cock near the boiler, using half inch hose pipe with a jubilee clip to secure it. Fit similar hose to another drain cock as far "downstream" as possible, leading to waste.
Run cleaning agent through system as required then, switch off circulating pump and open both drain cocks and flush through using mains water, until running clear. Turn off mains and close drain cocks. Run circulating pump and repeat cycle until satisfied system is running clear.
I appreciate that the mains water will fllow along the path of least resistance, so it is unlikely to flush out radiators, but the process of cycling between mains flushing and running the system normally should mean that the sludge and black water is eventually removed.
Of course, I may be overlooking something and I would be grateful for any advice.