Viessmann boiler failure

[ppppenguin]

Never had to drain any radiator either so no boiler sludge build up over the years.

Not necessarily. There's only 1 dose of limescale in the water but black sludge is a reaction between water and iron in the absence of air. It goes on continuously unless there's enough inhibitor in the water. I don't know how long inhibitor lasts before it loses its effect. If the system is vented (with header tank) as against pressurised there's also a certain amount of air and muck getting in that way. It's a prety small effect.