Vaillant boiler overflow leak?

My boiler pressure was low (less than 0.5) so after searching the net I found a video showing which screws to turn etc which worked but not before trying out other things including the Pressure Reducing Overflow (I didn't realise that's what it was for at the time). Although the valve is turned off there is a leak from the overflow pipe, so I'm now having to pressure up the boiler each day. What can I do about this new problem?