Fully pumped open vented heating system and position of cylinder return

have read

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

Harry