Okay, I've had a chat with my colleague who is a gynaecologist.
He agrees that combined oral contraceptives do not delay menopause and hence the follicles in the ovary, whilst not maturing to eggs, nonetheless continue to be 'consumed' by the ovary, despite suppression of ovulation. Therefore the eggs must be being recruited but then undergoing spontaneous involution (apoptosis) prior to maturation. How this is achieved is not clear.
My suggestion is that just as in a normal menstrual cycle when 40 or so eggs initially begin to develop and eventually involute leaving just 1 dominant follicle to ovulate, perhaps the oestrogen in the pill simulates that dominant follicle, causing those eggs that switch on at the start of the month to kill themselves.
Anyway, problem cleared up !
"I never forget a face, but in your case I'll make an exception"
- Groucho Marx