Much to my amazement, I discovered that the person I bought my house from had taken out an extended warranty, and Vaillant made arranging a visit from an engineer campletely hassle free. My own plumber wasn't a Vaillant specialist and wanted to replace that part (can't remember its name), from which the water is ejected down the copper pipe. Even I could see that there was really nothing that could go wrong with that, as it's a simple mechnism with a float inside. When the Vaillant guy turned up, he has a load of Vaillant-specific testing gear that the other guy didn't have, and was able to narrow the problem down far more efficiently than simply relying on a suck-it-and-see approach.
What I did find was that even after it was fixed, I was getting the sound of running water through the radiators, even though there was no air released from any of the bleed valves. It took about a month before the noise gradually went away of its own accord.
p.s. I also used the method of tying a freezer bag around the copper outlet pipe. It's a far more scientific way of going about it than wiping your finger on it.
p.s. I tried to quote Sofie's post, but can't find any way of making it appear. Just tried again and now it says multi-quoted, but it still doesn't appear.