Thanks audioguru
Works with just the voltage divider, R1=47k and R2=2.2k, so output impedance = 47||2.2 = 2.1k (please correct me if this is not correct).
DC Voltage measured over the electret is approx 1.6V - less than expected due to the high impedance of the voltage divider and relatively low impedance of the electret but it still seems to work just fine.
The only problem is audio feedback when the volume knob is turned up more than halfway (very painful the first time) - not sure if this is because the mic signal is too high, or the cheap computer headsets are not designed for isolation between the mic and the earpiece.
And the 10uF electro decoupling capacitor won't fit in the backshell of the XLR connector... might try and find a smaller one, tantalum etc.