Fully pumped open vented heating system and position of cylinder return

have read (http://www.gasman.fsbusiness.co.uk/fully_pumped_systems.htm ) that on a fully pumped open vented system the cylinder return must be the last conection before the boiler. Why is this? I have this system but the cylinder return is 3 radiators with trv's into the heating return side. Is this going to work or should I extend the return back to the boiler? (which will mean taking the floors up again). Or is there an easier way around it?
Thanks for any help

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