Yes, have a cold water tank in loft and a hot water cylinder. Don't have radiators however - have a warm air heating system in the house (yes - weird I Know!). Not sure how the air is heated - directly or via hot water. Not sure if this makes a difference re cylinder coil.
the gunk is almost certainly in the boiler/heating side (it will still feed the heat exchange coil in the hot water cylinder even though it is not feeding radiators). most likely is that the coil in the cylinder has ruptured.
it is possible that the gunk is getting into the cylinder from the CWSC in the loft if that is heavily sedimented, might be worth draining and cleaning it out.
if the coil has ruptured then its a new cylinder. you will have to clean and flush the whole system anyway so replacing the cylinder should solve the problem wherever the algae is coming from.
it is also worth noting that if this disappears (given how long ago this was posted), the problem is in fact still there and that the water has run clear as fresh water is drawn in to replace the treated water that you have lost.
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