Hi Mike
I think I sussed that one, there is a DC voltage at the electret which comes almost directly from the power source battery via a 4K7. If I remove that 4K7 that should solve that problem. I asked Velleman technical about that and they agreed with that. However, they also went to some lengths to emphasise the difference in impedance between the types of Mic and went on to suggest I test the circuit specified before I make any alteration, thus implying a problem will arise.
Should I ask you how you know ??