Thanks for your response.
Am I correct in understanding from your answer that there is no difference between a women on the pill and not on the pill. Would it be true too that if a women on the pill either forgot to do a moch after her hefsek or had a reason not to do one, for example for a few months after a natural child birth, this would also be ok and not affected by the fact she is on the pill and potentially more likely to spot.
Yes. There is no difference regarding the halachos of moch dochuk whether a woman is or is not on birth control pills.
All the best