Potterton Promax 15 SL?

New boiler 1yr old fitted by BGas who also charged for a power clean?. British Gas home care changed fan last month because boiler was maxing noises like a fan problem. Not cured so they called Baxi who changed the Heat exchanger finding a bit residue in it.
Now boiler quiet but rads gurgle at start of each cycle time no air just water out of air vents also has auto air expeler above the pump. Checked header tank and the water is black and stinks like a stagnant pond. Something floating on surface of water now out waiting drying to ananlize. The tank sits in the loft away on its own with nothing over or at the side that would have dropped in the water so assume this stuff could be a mix of muck in the system and chemical.