Hi Simmo
Secondary subfertility isn't actually all that uncommon unfortunately. You should be referred to a specialist at a subfertility unit.
With a cycle length of between 35-42 days these cycles could well be anovulatory ones (i.e ones in which ovulation has not occured). A "normal" cycle length is between 21 and 35 days, cycles over 35 days can often be anovulatory. It is certainly not the case at all the ovulation occurs 2 weeks before the start of the next period.
Ovulation is not an exact science and can occur earlier, later or not at all in any given cycle.
A day 21 test measures progesterone; its a standard test. This will determine whether ovulation has occured or not. However this is only part of the story. You need a day 3 test done too to see what your LH and FSH levels are like. LH and FSH are vitally important as they kickstart the ovulation process; if the levels of these two hormones are out then progesterone will be awry as well. If you do have a day 3 done (and I suggest this is done) the GP MUST compare the level of LH against that of FSH. Ideally these two are the same; an imbalance warrants further investigation.
BTW did you get multiple OPK postive results? If so this is a clue. What these kits measure is LH on the misleading principles that a rise in LH is immediately followed by ovulation and that women only have one rise in LH every month. Both of these are not true. If you are one of many women who are producing higher than normal levels of LH (a common problem with polycystic ovaries for instance) the kit reads that. Thus OPK's are not of any benefit to women with PCO.
LPD is controversial; some do not think it exists. Many GPs aren't aware of it and many can be dismissive of such things.
You need a referral to a gynae; you need a diagnosis of the underlying problem. Regardless of the progesterone result I would insist on a referral.
Do not be fobbed off!!
HTH
Attila