Valliant eco plus losing pressure - real mystery?

Hi all,

My boiler keeps losing pressure on a daily basis and has been doing so for over 2 months. At first it was a leak under the floor boards which i fixed however it is still losing consistantly. The Valliant engineer has been out several times and replaced several items like the prv and expansion vessel but still no joy. I switched the boiler off and sealed the system off from the boiler to confirm where the leak was and within 3 hours the boiler had still lost about .3bar pressure confirming it is a leak in the boiler. The prv valve is intact, expansion vessel recently been changed, no visable sign of leaks anywhere near the boiler which can lead to only one other possible answer - I am losing water through the condensor pipe (that is connected to the sink waste pipe) so its hard to check by just putting a bowl under it. If that is the case then how/why would i be losing pressure through there and how can I prove that is the problem so that the engineer can replace the necessary part (I'm thinking heat exchanger??) I'm a total novice however have become somewhat familiar with these boilers....

Many thanks