Thanks for the thought, but the circuit is unlikely to be incorrect. It consists of only 2 components
a) 5 volt power source from a high quality, ripple free DC power supply
b) the ZD1952 which only has 3 pins - Ground, Power and Output
The power supply is wired to the Ground and Power pins, while my measurements are being taken between Ground and the Output pin. Unfortunately I am away from home right now, so can't follow up with the advice to check the CRO, but that seems to be the most likely explanation.