It does not matter which rad it was added to - it's all one circuit of water whether it is used to heat rads or to heat water in a cylinder.
To be fair, to jamesdean, you're missing the point of his post. He believes that it used to be commonplace to connect a rad on a secondary circuit. However, if that was the scenario, then the X400 wouldn't be in the other radiators.
The OP has said that there's pollution between primary and secondary water, therefore the crossover is in the cylinder.