I had the same thing changing a diverter valve on an old Greenstar 28 CDI. Put it all back together and it leaked on the cold water inlet O ring seal. Second attempt at cleaning it sorted it though. Used a small angled wire brush and a small old screw driver which I bent at a 90 degree angle at the end. Works a treat at getting the lime scale out of the grove the O ring sits in. Took much longer than the 2 hours I quoted though and now the flow/return valves are both leaking since I touched them to isolate the boiler. Worcester now called in for a fixed price repair as it's for a big repair company. Takes hours to replace them.