So, it's been a few months and I thought that I should come back and update. It would be great if others do the same because it seems difficult to get a range of views about post-mirena TTC.
My first four cycles were 23 days, 29 days, 27 days and 25 days long. It became apparent that I was ovulating quite late on in my cycle and only had a luteal phase of 7-9 days. I did a bit of digging around about a short luteal phase and decided to try high strength vit B6 as it can help. I read that some women have good effect from 50mg daily and others need 100mg. I also heard that B6 is better absorbed with other B vits so DH got me a B100 complex from Holland and Barrett that is a slow release one to help absorption. There is also some evidence that mirena (like lots of other hormonal birth control methods) uses up a lot of vit B 6 so it's not unusual to end up with a short luteal phase afterwards. I have had been very deficient in the past and all of a sudden realised that I had the same symptoms again.
I also read that vitamin D is important for oestrogen production and that there may be some links between vit D deficiency and poor implantation and early miscarriage. Pregnancy and BFing deplete vit D so I wouldn't be surprised if I were deficient having done a 5.6 year stint of pregnancy and BFing for our 3DC.
Anyway, the upshot is I am now on cycle 5, 11 dpo and got a BFP this morning! I've only been taking the vits from CD2 but I am pregnant! Now I just have to hope for a sticky bean I should add that all of my other pregnancies have been conceived on cycle one but, obviously, I have no proof that it took 5 cycles because of mirena.