Here in the US it is recommended that we drain water from the bottom of our hot water heaters at "least" once per year (I try to drain mine 2 or more times per year), because calcium and other minerals precipitate to the bottom of the tank. The sludge that comes out if not done yearly can be a viscous, grey to black color and not good for human, or fish consumption.
But in the US, these water heaters are under pressure, so even if not done yearly the sludge is never as bad as the yours. This mineral build up could be host to all your problems.
My girlfriend, also a Brit says your attic water heaters are gravity fed, and under only slight pressure, so I would imagine sludge is more easily built up and sounds like more frequent cleanings are needed. The new line you added may have upset the balance enough to stir up the rubbish that caused your bloom, and or, effected your bio filtration.
, need it sorted asap going Away on Tues 