Condensate trap leaking on Heatline Vizo boiler?

The condensate trap has a steady drip from the side when the hot water is running. The plumber who fitted it thinks there is an air lock in the drain pipe, but I've taken the pipe off and blown it through (as he suggested). I've also taken the bottom off the trap, cleaned it, emptied it and re-filled it with 50cc of water (as per manufacturers instructions). Its still leaking. Any suggestions?