On investigating I discovered that it was the geyser valve leaking -- the one we had had replaced 16 months ago. As I noted at the time (29 Jan 2008):
Mobile plumbers came, replaced geyser inlet valve.
400kpa Apex pressure valve R625.78
Total cost R1133.78
I also noticed that the drip tray installed by the same plumbers 2 years previously did not catch the drips, but that the water had leaked into a cupboard and wrecked a lot of stuff. So they came and made the outlet of the drip tray go to the front of the house instead of the back. I still didn't trust them, however, so the cupboard has stood empty for 16 months, with the stuff all over the floor in my study, and down the passage. Just as well, because what wasn't wrecked last time would have been wrecked this time -- files, papers, photographs, CD-R discs and stuff like that.
Are there any fundis out there who know if there is any alternative to these very expensive geyser inlet valves that cost R625 and last for only 16 months before the threads attaching them to the pipe dissolve and water drips into the house? We had to have several replaced over the years, and whether the threads are made of brass or plastic seems to make little difference -- they dissolve and leaks result.