There's plenty of X400 in there, I doubt filling the system with neat X400 would make the slightest difference. Manifold may be blocked but that would only affect the rads, not the hot water.
Just to rule them out, are both motorised valves open? Be pure coincidence if both have failed but not impossible. If you open manually on the levers, should be some resistance as you are pushing against a spring. If that still doesnt help, I would remove one of the head and just check the spindle is turning and actually opening the valve. If the motor is working as it should then boiler and pump will be energised, but if the water cant get past the valve(s) (as it's broken/stuck shut) then system wont work....
Just to rule them out, are both motorised valves open? Be pure coincidence if both have failed but not impossible. If you open manually on the levers, should be some resistance as you are pushing against a spring. If that still doesnt help, I would remove one of the head and just check the spindle is turning and actually opening the valve. If the motor is working as it should then boiler and pump will be energised, but if the water cant get past the valve(s) (as it's broken/stuck shut) then system wont work....