Update: I have since tried the following combinations of C and R - (10uF & 680 Ohms) and (4.7uF & 1.5 kOhms). There is no change whatsoever in the response - it still works only between 100% and 50% duty cycles (corresponding output voltage is 5V to 0V).
CrossRoads - I'll try your suggested values next. Thanks for your input.