dizzy, I had several miscarriages in my late 30s/early 40s - all natural pregnancies. I also had one IVF and several stimulated cycles (Gonal-f, not clomid) none of which worked for me. I then, after a lot of soul searching, tried donor eggs, in case the issue was egg quality (my fsh was about 10-11 at the time), got pregnant, and miscarried yet again - at that point, I saw a different doctor and was finally told I probably had an immune issue (this was all new at the time) which was causing the miscarriages.
By the time we had got our heads around all that, my FSH was higher (17) and my response to meds lower: I was 42 and had been TTC for years and years. So as it was our last ditch attempt, more for closure and peace of mind than with any hope of a baby, we threw the book at it: immune treatment (heparin, aspirin, steroids) and donor eggs. I have the most beloved dd from all that help...but as you can see, it was a LOT of help. And to my ongoing amazement, I'm expecting a second (and last!) dd soon via the same route.
An fsh of 20 is pretty high, but its possible to have a one-off bad month - even if not on anything. If your usual is 13...most clinics that I know of would say that your best chance of conceiving successfully would be naturally or by IUI (IVF bashes already fragile eggs around quite a bit). On the other hand, most will treat women - especially under 42 - with an FSH of up to 15. They vary a lot though, if you really want to try IVF it would be worth ringing around a couple of other clinics to see what their fsh cut-off is.
Also, some clinics are keen to avoid Clomid with older women - it can thin the uterine lining, and has more side effects than the 'big gun' meds. But there are pros and cons, of course.
HTH a little...its such a painful journey, wanting to be pregnant and not being able to be. And I don't think its that much less painful if you already have a child - just different. Wish you loads of luck!