Worcester 240 slow pressure rise?

Hi..I have recently replaced two major parts in my Worcester 240: "Diverter Valve".... during the summer 2009 (radiators heating up when i had demand hot water only). "Expantion Tank"... due to pressure rising over time. (drained air from vessel air & water came out) now December pressure rising again but at a faster rate est 4 days no problem with expantion vessel, at 2.8bar i drain excess water from system via the last radiator in the loop with all others locked off. could i now have a problem with the hot water seeping into the heating system via the heat exchanger, there are signs of rust in the water i drain out.