Just thought I'd share my story. I have always had long cycles but fell pregnant first cycle after coming off the pill (it took five weeks to release an egg after coming off the pill, so it would have been a seven-week cycle). That was in January. But I miscarried at 13 weeks in March (baby died at 10 weeks but I didn't start bleeding till 13 weeks).
I had a D&C after my MC and then had a bleed (very light, didn't feel like a proper period) five weeks later. My next period was six weeks after that (which did feel like a proper period). My next cycle was very long and I went to the doc 10 weeks into it (there was still no sign of it). He sent me for blood tests (FSH, LH, thyroid and prolactin) but they came back "normal". But my period still hadn't come so I went back to see him and he basically just said it was probably stress causing the long cycles. I didn't agree cos they have always been long so I asked to be referred for "Amenorhoea" (I have private medical insurance and it covered it because Amenorhoea is not "fertility").
The GP didn't want to refer me and I got the impression there was no way he would have done on the NHS.
Anyway, I went to see the same surgeon who did my D&C and he told me that long cycles rarely end in pregnancy because by the time the egg is released it is not viable. This may also have been why I miscarried.
He gave me progesterone to make me have a period and then prescribed me the fertility drug Clomid to make my cycle regular.
He also sent me for more blood tests (they have to be done on day 1-3 of your cycle so the ones the GP sent me for were irrelevant - he was probably just trying to get rid of me).
So now I am on Clomid, CD19 of my first cycle and my temp charts show I ovulated on Day 14!
So my advice to anyone with long periods is just keep going back to the GP and demand to be referred. If you can afford it - go private. I only needed one appt with the gynae which wasn't expensive although he did say that as I had been pregnant before it was obvious my tubes etc were OK. If you went to see him cold he may wish to do other tests before prescribing Clomid.
If you are under 30 then I would imagine they want you to be ttc for a year before doing anything. But it doesn't hurt to ask - and keep asking!