I have just serviced the diverter valve with a service kit from vokera, all is now working correctly but I cannot seem to stop a leak coming from the top edge of the diverter valve front cover. When I put a second cover gasket in, the leak did not appear straight away but only later when the boiler had heated and cooled a couple of times ( this is what happened the first time I put a gasket in). Is it possible the front cover could be warped. Appreciate any help.